Quicksilver for Mac OS X is a launcher very much based on keyboard input.
It works by bringing up an alt-tab-style menu via a definable hotkey combination and a search algorithm for finding items matching your input, which works almost instantly. Define which parts of your system are searched, so it’s fine to just serve for starting applications.
You can get much deeper into it, though. Do not only display applications, but any objects. Do not just launch, but use any other action in an application or on an object. Query an online dictionary for input text. Email a paper to your friend. Do it all with just a few keystrokes.
You can find the freely distributed software here.
Dan Dickinson’s blog presents a lovely introduction to using the code.