I *really* love the modding communities for both Baldur's Gate 1 and 2. Some of the content that's been delivered by those dedicated fans is of such high quality that it puts many retail works to shame in terms of dialogue and story-telling. Not to mention the engine improvements and tweaks which have been made over the years. I literally cannot play Baldur's Gate 1 without certain mods anymore such as the Tutu mod and the BG1 NPC Project.
Then there's the huge library of fan made campaign for Neverwinter Nights 1 and 2, many of which are of such scope and scale they make the official expansions seem petty by comparison. No, really! Not only that but many of the stories told through these fan-campaign are better than most retail games could even hope to boast, it's damned impressive. I've easily sunk more hours into fan project for both NWN1 and 2 than I have playing what came with the games themselves and enjoyed many of those fan projects more as well, Mask of the Betrayer being the sole example otherwise.
As for fun on projects, I've really gotta commend the Thief community as well. T2X is a fan made expansion for Thief 2 which boasts professional levels of quality. Not just new maps, but a whole new story, gameplay mechanics, voice acting, and even cinematics. If you like the old Thief games, it's pretty much mandatory you download and enjoy T2X.
More recently I've really been enjoying the Mechwarrior: Living Legends mod for Crysis. It really is an amazing total conversion, taking a first person shooter that boasted a few neat mechanics and transforming it into a multiplayer giant-stompy-mech simulator that's just damned fun to play. If development on this mod continues to go as well as it has been, Mechwarrior 5 is going to have some stiff competition. Seriously.
