Two fairly small mods I love are unavailable on most versions: Carry-On is only for recent versions of MC, and Peach Dudes is only for Beta versions.
I'd love to have both of those available on MC 1.2.5!
If you're wondering why I'm using that version, the answer is simple nostalgia! The Lasers Mod and computers from Red Power 2 work very well together, and I feel I didn't use them to their full potential. I know Red Power updated to 1.4.7, but the Laser Mod (and the ability to see through the portals from the Portal Gun mod) did not.