Is it possible to extend spotlight's search feature to support additional commands?
For example, is it possible to define custom web search aliases, meaning I could type yt and it would provide results from youtube.
Alternatively, is it possible to add additional system commands were I could type sl (or some variant) to put my computer to sleep.
In the meantime there is Flashlight (The missing plugin system for Spotlight) http://flashlight.nateparrott.com/
This is expandable with Python scripts.
UPDATE Oct 2016 It seems that Flashlight is not longer maintained. The last update was more then a year ago. Check the repo here: https://github.com/nate-parrott/Flashlight
UPDATE Mar 2017 There is a fork which is still maintained: github.com/w0lfschild/Flashlight – Thanks @amoebe