Mac OS X Leopard

Mac OS X Leopard (version 10.5) is the sixth major release of macOS, Apple's desktop and server operating system for Macintosh computers. Leopard was released on October 26, 2007 as the successor of Mac OS X 10.4 Tiger, and is available in two editions: a desktop version suitable for personal computers, and a server version, Mac OS X Server. It retailed for $129 for the desktop version and $499 for Server. Leopard was superseded by Snow Leopard (version 10.6) in 2009. Leopard is the final version of macOS to support the PowerPC architecture as Snow Leopard functions solely on Intel based Macs.
According to Apple, Leopard contains over 300 changes and enhancements compared to its predecessor, Mac OS X Tiger, covering core operating system components as well as included applications and developer tools. Leopard introduces a significantly revised desktop, with a redesigned Dock, Stacks, a semitransparent menu bar, and an updated Finder that incorporates the Cover Flow visual navigation interface first seen in iTunes. Other notable features include support for writing 64-bit graphical user interface applications, an automated backup utility called Time Machine, support for Spotlight searches across multiple machines, and the inclusion of Front Row and Photo Booth, which were previously included with only some Mac models.
Apple missed Leopard's release time frame as originally announced by Apple's CEO Steve Jobs. When first discussed in June 2005, Jobs had stated that Apple intended to release Leopard at the end of 2006 or early 2007. A year later, this was amended to Spring 2007; however on April 12, 2007, Apple issued a statement that its release would be delayed until October 2007 because of the development of the iPhone.
New and changed features.
End-user features.
Apple advertised that Mac OS X Leopard has 300+ new features, including:
Security enhancements.
New security features intend to provide better internal resiliency to successful attacks, in addition to preventing attacks from being successful in the first place.
Security features in Leopard have been criticized as weak or ineffective, with the publisher Heise Security documenting that the Leopard installer downgraded firewall protection and exposed services to attack even when the firewall was re-enabled. Several researchers noted that the Library Randomization feature added to Leopard was ineffective compared to mature implementations on other platforms, and that the new "secure Guest account" could be abused by Guests to retain access to the system even after the Leopard log out process erased their home directory.
System requirements.
Apple states the following basic Leopard system requirements, although, for some specific applications and features (such as iChat backdrops) an Intel processor is required:
Leopard’s retail version was not released in separate versions for each type of processor, but instead consisted of one universal release that could run on both PowerPC and Intel processors. However, the install discs that ship with Intel-based Macs only contain Intel binaries. 
Processor type and speed are checked during installation and installation halted if insufficient; however, Leopard will run on slower G4 processor machines (e.g., a 733 MHz Quicksilver) if the installation is performed on a supported Mac and its hard drive then moved to a slower/unsupported one (the drive may either be an internal mechanism or a Firewire external).
Supported machines.
Leopard can run on the later flat-panel iMac G4s, the iMac G5, iMac Intel Core Duo and iMac Intel Core 2 Duo, PowerBook G4, Power Mac G4, Power Mac G5, iBook G4, MacBook, MacBook Pro, MacBook Air, Mac Pro, Mac Mini, Xserve, Xserve G5, Xserve RAID, Macintosh Server G4, and later eMac models. Leopard can run on older hardware as long as they have a G4 upgrade installed running at the 867 MHz or faster, have at least 9 GB free of hard drive space, 512 MB RAM and have a DVD drive. Leopard however will not run on the 900 MHz iBook G3 models even though they exceed the minimum 867 MHz requirement. This is due to the lack of AltiVec support in the G3 line of processors. Leopard can be "hacked" (see below) to install on these G3 and pre-867 MHz G4 machines but the system may behave erratically and many of the programs, features, and functions may not work properly or at all. As of mid-2010, some Apple computers have firmware factory installed which will no longer allow installation of Mac OS X Leopard. These computers only allow installation of Mac OS X Snow Leopard. However, some computers (such as the 2011 model of the Mac mini) can have Leopard installed on them without hacking.
Usage on unsupported hardware.
Some ways of running Leopard on certain unsupported hardware, primarily PowerPC G4 computers with CPU speeds lower than the official requirement of 867 MHz, have been discovered. A common way is use of the program LeopardAssist, which is a bootloader similar in some respects to XPostFacto (used for installing earlier releases of Mac OS X on unsupported G3 and pre-G3 Macs) that uses the Mac’s Open Firmware to tell Leopard that the machine does have a CPU meeting the 867 MHz minimum requirement that the Installer checks for before installation is allowed to commence, when in reality the CPU is slower. Currently, LeopardAssist only runs on slower G4s and many people have installed Leopard successfully on these older machines.
Users who have access to supported hardware have installed Leopard on the supported machine then simply moved the hard drive to the unsupported machine. Alternatively, the Leopard Installation DVD was booted on a supported Mac, then installed on an unsupported Mac via Firewire Target Disk Mode. Leopard is only compiled for AltiVec-enabled PowerPC processors (G4 and G5) though, as well as Intel, so both of these methods will only work on Macs with G4 or later CPUs. While some of the earlier beta releases were made to run on some later G3 machines (mostly later 800–900 MHz iBooks), no success with the retail version has been officially reported on G3 Macs except for some later iMacs and "Pismo" PowerBook G3s with G4 processor upgrades installed.
For a number of months after Leopard's release it appeared that the only G3 Macs on which Leopard could be run were those with both an aftermarket G4 processor and an AGP graphics card, as failures with the OS partially booting before crashing were reported on older Macs such as the original tray-loading iMacs and the Beige and Blue & White Power Mac G3 (all with G4 upgrades as Leopard will not even begin to load without one) whereas it would boot fine on newer Macs where the Installer restriction had been circumvented. However, more recently it has been reported that with some more work and use of kernel extensions from XPostFacto, Tiger and beta builds of Leopard, the OS can be made to run on G4-upgraded Macs as old as the Power Macintosh 9500, despite the lack of AGP-based graphics. While Leopard can be run on any Mac with a G4 or later processor, some functionality such as Front Row or Time Machine fails to work without a Quartz Extreme-capable graphics card, which many of the earlier G4s did not include in their factory specification.
Since Apple moved to using Intel processors in their computers, the OSx86 community has developed and now also allows Mac OS X Tiger and later releases to be installed and run successfully on non-Apple x86-based computers, albeit in violation of Apple's licensing agreement for Mac OS X.
Packaging.
The retail packaging for Leopard is significantly smaller than that of previous versions of Mac OS X (although later copies of Tiger also came in the new smaller box). It also includes a lenticular cover, making the "X" appear to float above a purple galaxy, somewhat resembling the default Leopard desktop wallpaper.
Compatibility.
After Leopard’s release, there were widely reported incidents of new Leopard installs hanging during boot on the blue screen that appears just before the login process starts. Apple attributed these problems to an outdated version of an unsupported add-on extension called Application Enhancer (APE), from Unsanity which had been incompatible with Leopard. Some users were unaware that APE had been silently installed during installation of Logitech mouse drivers. However, only the users who did not have the latest version of APE installed (2.0.3 at that time) were affected. Apple published a knowledge base article on how to solve this problem.
Google announced that the Chrome browser will be dropping support for Leopard starting with Chrome 21. By that time Chrome will no longer auto-update, and new Chrome installations are not allowed. Their rationale for removal of support is that Leopard is an "OS X version also no longer being updated by Apple."
Firefox also dropped support for Leopard after it shipped Firefox 16 in October 2012. TenFourFox is a port of Firefox for the PPC architecture, released after Firefox dropped support for Leopard.