Today Google released Quick Search Box for Mac, which is developed by the same person who wrote Quicksilver (Nicholas Jitkoff works for Google now). As expected it works more or less the same way quicksilver does. According to Google, QSB is an evolution of Google Desktop for Mac. If you are a long time Quicksilver user you will fit right at home with QSB, minus the large number of plugins and configuration options with Quicksilver.
Currently QSB only has one twitter plug-in, but it is expected to get more plug-ins as more people starts using it.