I am running CS3, waiting for CS4. I get the old version from my dad when he upgrades to CS5. CS3 is great for what I need and I know the images my dad printed while using CS3 are fantastic. CS4 does have some improvements that I can use (Although I really like the new CS5 features). I can't really give advice on any other program.
If you don't know anything about PS or other programs, you can tryout
GIMP. It's free, runs on most platforms and is surprisingly robust these days. I used it many years ago on Linux and had good things to say then. I believe the interface is a little friendlier today too.
I forgot about PSP as Trekkor mentioned.