Digital Vibrance (NV)
Colors are more vibrant. Very good to make colors rich and warm. Works everywhere.
Image Sharpening (NV)
Everything on screen becomes sharpen, including textures. Excellent for Quake 4 or other games where you see lots of metal objects. Too high setting may display shimmering on certain objects. Works everywhere.
Temporal AA (ATi)
Perfromance free FSAA multiplying. Works only when framerate is synced with refresh.
May be useless if you have display with refresh of 120Hz+ and some (too) weak card that can't output such framerates.
Not sure if it isn't limited only to Direct3D...
SmartShader (ATi)
Custom post-processing effects. Default are mostly useless except Sharpen (similar to NV's though it works only under OpenGL) and Blur (creates fancy Depth of Fieled effect, again only OpenGL). All extra ones (home made) can only support OpenGL and only very good effect is HDRish effect (works excellent in Doom 3,Starsiege and Rune/UT99/Deus Ex).
TruForm (ATi)
Objects smoothing by multiplying polygons. Works correctly only on supported games, ATi discontinued this feature in Catalyst 5.12 if i'm not mistaken.
These are one that come in my mind at the moment.
Though i wonder why chip vendors don't impliment more of features like Digital Vibrance that is proven to be very useful for many years (and a major reason for many users to go with NV). SmartShaders are also very cool thing but since all good effects are limited to OpenGL only this feature becomes completely useless since they become useful only in id Software games... Funny decision...
We have lots of horsepowers under the hood, yet we don't see much of such goodies