Okay, this one is going to appeal to a very small audience (command line jockeys), but I suspect it will make those few people very happy. Vi Input Manager is, well, an OS X Input Manager that "patches the Cocoa Text System to add a Vi-like command mode." So in any Cocoa application with a text field (e.g. a form in Safari) you can enter "command-mode (typically, by hitting escape in Vi), [and] ordinary Vi commands can be typed and the text field will be updated accordingly." So basically, if you've invested years getting all those Vi commands in muscle-memory now you can capitalize on it in your favorite Cocoa applications (emacs fans don't shoot the messenger).
Vi Input Manager is Open Source and a free download, but donations are requested.