Mac OS X startup config
Since OS X no longer has extensions per se managing them is not necessary. (Thankfully)

I still have horrifying memories of managing different extensions which had varying degrees of incompatibility often requiring different load orders and multiple reboots to straighten out.

Here is an excellent article, although old, that still outlines the OS X system startup process:

http://osxbook.com/book/bonus/ancient/whatismacosx//arch_startup.html

The closest thing to an extension manager is this freeware utility that manages items in your Library folders, allowing you to enable/disable them. I don't use it myself and have no idea if it still works with recent Mac OS X releases.

http://s.sudre.free.fr/Software/Diablotin.html