Fallout 2
A turn-based RPG with a gigantic variety of sidequests, interesting things, and random sh*t.
Includes the ability to play as a developmentally delayed person, to have a gay marriage, and to murder small children if they annoy you too much.
Now available on Steam, as well as GoG.com (http://www.gog.com/en/gamecard/fallout) and you can also buy the entire trilogy directly from Interplay, who as I understand it are currently working on a mmo version of fallout (and so could probably use your support) [1]
A handy directory of almost(?) all the mods available for fallout 2 is available at No Mutants Allowed: http://www.nma-fallout.com/forum/dload.php?action=category&cat_id=15
the Fallout 2 Restoration Project is strongly recommended- it includes many mods that make the game more accessible (easily set the resolution for example) while only adding content that was originally planned to be included by the devs but were cut for time/budget reasons.