I've been really impressed by Aza Raskin's Ubiquity add on for Firefox, which I've been playing with a lot lately. It completely makes up for the less than exciting experience I had with Enso, which never came close to a Quicksilver experience on Windows. Ubiquity is pretty much the Quicksilver experience brought to your web browser. A smart tool that is summoned by a keyboard shortcut, and provides shortcuts to frequently used tools, and contextually relevant paths to using selected content in the page. Raskin talks more about the app on his site.