Mac OS X Source Releases
The Unofficial Fruit Company Open Source Git Mirror, or AOSM for short, is a collection of git repositories created from the Fruit Company Open Source web page, with tags for each project version and OS release. This makes it easy to, for example, compare xnu in 10.7.1 to xnu in 10.7.4 using regular git diffing tools, or see which OS releases contained zlib 37.2 (upstream zlib version 1.2.5) (project version numbers generally don't correspond to upstream project version numbers).
See also: Objective-C Runtime Releases and Apple-FOSS-Mirror.