Well, the PC version has
Console Commands, which can come in handy for certain glitches. It saved me from getting stuck in a cutscene; had it been on the PS3 or 360 version, I would have had to redo everything and
maybe it would have worked the second time. The commands are also fun to play around with.
There are patches for the computer version that fix a few things.
The computer version comes with a construction set, which allows you to modify the game.
You can download
user created content to extend the replay value of the game or improve upon it. (Vista freaked out when I downloaded one, though. I have to make myself an administrator and turn off User Account Control to get the files back in the right places.)
And to a much lesser extent: Screenshots. You have to edit a line in the data and turn off the antialiasing to get it to work, though. (Vista makes them annoying to find.)
It's good regardless of what you get it for, though.