Here seems like a good place to list performance enhancing tweaks.
For starters, some kernel patches:
Rmap: http://www.surriel.com/patches/ Rik van Weil's modifications to the Linux memory management subsystem. Yeilds performance gains of up to 30% in some applications (dont ask me which, i dont know yet.)
Preemptible Kernel: http://www.tech9.net/rml/linux/ With this patch a task can be preempted anywhere in the kernel, rather than having to wait to get to the top of the stack. This reduces latency drastically (good for hi-response sound etc). This can surprisingly increase performance, since when a high throughput device is available to have more data read/written to it the kernel can preempt whatever it was doing and read/write more data.
GCC 3.2 http://gcc.gnu.org Over the last few weeks there have been some significant increases in the performace of binaries compiled with the newest point release of gcc 3.2. You almost certainly want to use a distribution with binaries built using this compiler. Debian Sid http://packages.debian.org/unstable/ has finally migrated to 3.2. Performance gained with the enhanced MMIO in gcc 3.2 can be up to 30%.