I don't do any myself, but I've been impressed with the following works by other people:
Oni - updates for character models, also character skins and better A.I
Baldurs gate I and II: made them into one sprawling unabridged epic RPG, also updated the gameplay mechanics of I with the systems of 2.
FFVII: character models improved along with NPC's and other stuff (kinda irrevelant given recent E3 news but hey...)
KOTOR2: all that added content and some cutscenes made it a much better experience.
Mass Effect 3: that 'shepard lives' ending mod.
I haven't played all of these but I've seen some of them on youtube along with this
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