8800 GT as part of a full build with a C2D that replaced a P4 with a 6800 GTS. That computer absolutely screamed compared to what I had before.
Other than that, the Vapor-X 290 I replaced a 970 G1 Gaming with. Cost less by a good amount, and the drivers have been rock steady unlike the 970, and the 290 paired with a $600 refurb XR341CK has been phenomenal value at 3440x1440. It wasn't a performance beast by the time I got it, but it's a lot of performance and a really great experience for the price. Only downside is it is a bit of a room heater.