Video Games With Staying Power

I can't really justify talking about this as a "Summer Of Free" segment, but I am feeling absolutely blown away by the amount of game contained in Neverwinter Nights 2.(And the original Neverwinter Nights as well.) And not just length of playing time but variability, given the huge number of adventure modules, campaigns and other goodies made by users that are available for free online.

My wife got me NWN2 for my birthday in April and I don't think its an accident that I've been spending a LOT less from my "fun money" budget, resulting in a pretty big savings pool. (At least big for me. Money burns a hole in my pocket.)

So I'm curious what other video games (on any platform) you all have discovered that, compared to the initial investment, give back an enormous amount of play time.

Anything come to mind?

Well, I'm probably in the minority here, but I've played through the Halo games at least twenty times a piece, and Bioshock's the same... As far as adventure games, I STILL play Legacy of Kain: Soul Reaver for the PS1 from time to time (and go through it's sequels as well)... The Onimusha games all the way through Dawn of Dreams on the PS2 also get a lot of play once or twice a year... I also get a lot of play out of the Fable games and Fallout 3.

I've got to say Kingdom Hearts is something I've play through a lot and with 1.5 coming out this Sept. I'm going to be spending my time playing through Kingdom Hearts, Chain of Memories and watching the 358/2 days "movie" trying to get all of the trophies.
Which brings me to my 2nd thing I think trying to get all the Trophies and Achievements in a game is a nice challenge and good way to spend some extra hours in your game