Work has a large deployment on AIX, so I've had to learn a little about AIX. I keep myself sane by installing a lot of Open Source tools on my AIX boxes.
IBM have a good selection of packages available, though they are very conservative about issuing updates so versions are often quite old.
Groupe Bull offer bullfreeware.com. I find this rather more hit and miss. Their Gnome packages are good, much more up to date than IBM's 2.4.0 offerings. Though as of mid November 2007, still at 2.14, with 2.16 in test (and not working when I tried a few days ago).
I also have a few packages I've either made myself from scratch, or updated versions of the IBM packages. They are on my Aix Packages page.