genpatches is a patchset applied to many of Gentoo Linux kernel packages. It aims to support the entire range of Gentoo-supported architectures.
Each patchset is based on the initial stable release of the kernel released at kernel.org (e.g. 2.6.10, 2.6.11, 2.6.12).
The patchset is split up into two parts: base and extras. base contains bug and security fixes, while extras includes some extra hardware support and some feature patches.
genpatches is mainly produced for gentoo-sources, which is the only kernel to include both base and extras.
genpatches is maintained in Gentoo SVN. Unfortunately public access is not available, however it is browseable online and the trunk contents are regularly mirrored here.
genpatches is not specific to Gentoo at all, it can be viewed as a general purpose Linux patchset focusing on stability. Some other non-Gentoo projects use genpatches too, and that great. If you want to use genpatches in your distro/project, please feel free, and do let me know.