Quicksilver is similar to Alfred in intent and quite featureful/customizable. It is open source. https://github.com/quicksilver/Quicksilver
It's too late for me anyway - I have moved to Alfred, the new kid on the block at the time, and like it much more (much less clutter). Plus, I can't trust the QS developer anymore after he's abandoned the project once.
I know because I regularly see "Quicksilver has been updated" messages ;)