Cydia. Love it or hate it, it's one of the pillars of the jailbreak community. When Cydia acquired Rock a few months ago, Saurik made the promise that he would work on improving Cydia's speed, which, let's be honest, leaves a lot to be desired.
So we knew for a while that it was being worked on by Saurik and Chpwn, but with the release of the recent jailbreak for iOS 4.2.1, we've had a taste of where Cydia is going, and I like that a lot.
While the changes in the new release of Cydia aren't much "esthetically visible" as most of it happens under the hood, if you have the new Cydia on your iPhone, you must have noticed some speed improvement...