Where can I download Filemerge - the app for comparing two tools and merging them?
FileMerge is part of Xcode 4.x by Apple which you can download from the Mac App Store.
The following is taken from the description:

Xcode is now distributed as an application, rather than as an installer. This change enables Xcode to be updated directly from the Mac App Store.

Includes SDKs for Mac OS X 10.7 Lion and iOS 5
New code signing workflow supports signing OS X apps with Developer ID
Adds installation of command-line tools to the Downloads preference pane
Open Developer Tool menu launches Instruments, FileMerge, and other tools
ARC migration tool converts both retain/release and garbage collected code
Additional bug fixes and stability improvements


But FileMerge.app is now embedded inside the Xcode.app package and is not easily accessible as a standalone tool... Unless you do this:

Browse to /Applications/Xcode.app
Right-click -&gt; Show package contents
Contents -&gt; Applications
Right-click FileMerge.app -&gt; Make Alias -&gt; Enter password
Copy the Alias to somewhere, like /Applications
Delete the original Alias

Now you can use as a standalone tool, as before!