Launchy, Skylight, Quicksilver

Similar to Quicksilver for Macs, Launchy does just about the same and is great if you are not using Windows Vista and need a quick launch/search feature and do not use any type of desktop search or indexing program such as Google Desktop. If you have Vista, this is still useful as it allows for more advanced features. Another one that looks even closer to quicksilver is Skylight.