You've mentioned Fallout 3 which is probably the best of the current Gen. Someone mentioned Oblivion which is the same engine as Fallout and very similar.
Some other suggestions include:
#1: Sacred 2: Not a game for everyone, despite a few problems and some bad voice acting Sacred 2 is like a combination of Diablo and Oblivion. It plays from a top down, real time perspective. You have a number of character choices, and some greatly varied ways of building each one, and tons of differant loot that drops to find and collect. The big thing that makes this game though is that unlike Diablo which followed "Chapters", Sacred 2 uses an open world format where you can pretty much go wherever you want. The world is also full of quests outside of the main plotlines (there is a very weak central story, and a class based story for each character type which is slightly better in most cases), these quests range from very good, multi-part stories which can be fairly epic in scope, to very simple "go fetch this, or kill X number of monsters" type quests. If it's your type of game this can easily suck up hundreds of hours of your time. What's more you HAVE to replay it to see everything in the game as a lot of stuff won't drop until you hit the higher difficulty levels (which are only unlocked by beating the game).
#2: Monster Hunter Freedom: This is a PSP exclusive, I give it credit for replay value simply because you'll do the same hunts again and again quite a bit. Due to the controls I've never gotten into it (and yes I know some people have learned to use them far better than I have), but it's another one of those games that can suck up hundreds and hundreds of hours of time to get the most out of it. They recently released a new game in the series (Monster Hunter Freedom: Unite) a few days ago. Still trying (very pathetically) to unlike Sword Saint Piercing in part 2 however so (heh) we'll see if I ever play the new one (which I have sitting on a pile of other games, having received it as a gift much like Red Faction: Gueriella).
In general though I can't think of many games that scream "replay me" other than maybe Sacred 2. Games that will take a lot of time to complete, yes, but not a lot that beg to have you retread the same things a ton of times.
