Ignore dfusco, that's not a processor.
It really depends on your budget but here are few that are worthy contenders.
Core i7 920
Core i5 750
AMD Phenom II X4 955 Black (can easily be overclocked to 3.8Ghz with the right cooling)
AMD Phenom II X4 945 (If you don't want to overclock)
AMD Phenom II X3 720 Black (Easy to overclock)
AMD Phenom II X3 710 (If you don't want to overclock)
AMD Athlon II X4 620
AMD Athlon II X3 435
Personally, I am upgrading soon and I don't have a lot to spend but I'll be getting the Phenom II X4 955 Black Edition (then I'll overclock it to 3.8Ghz).