Better gdb support (gdb comes with the base system, so I assume it's not off-topic). I miss:
- reverse execution mode - I think that's currently tied to i386-linux
http://sourceware.org/gdb/news/reversible.html
- following forks() - that's currently tied to linux and hpux
http://sourceware.org/gdb/onlinedocs/gdb/Forks.html