Debian GNU/Linux stuff
Here I will mention some nice software for Debian GNU/Linux.
pf-kernel: more responsive GUI
pf-kernel is a Linux kernel that includes a specific set of patches ontop of mainline Linux that for various reasons haven't been merged in mainline Linux. It includes the BFS scheduler by Con Kolivas, and BFQ I/O disk scheduler. With these improved schedulers, I can now run videos and play 3D games on my laptop (Intel Core 2 Duo) where before the machine would freeze frequently. The pf-kernel also includes Tux-on-Ice, which is an alternative hibernate implementation, but I haven't been able to get that completely working yet.
Debian packages for x86 and amd64 by Ivan Svarkovsky
infinality: nicer-looking fonts
infinality includes patches to freetype and fontconfig that improve font rendering. It includes customization features so you can choose to make the fonts render more like they do on e.g. Windows or Mac OS X.
Debian packages for amd64 by Xiao-Long Chen and hadrons123