Speaking of Spotlight add-ons, Nate Parrott has a workaround/hack/plugin system that extends the functionality of Spotlight in Yosemite to include Wolfram Alpha searches, weather reports, and more.
Via Federico Viticci, who stumbled across a few bugs when using it:
I got similar results with weather and Wolfram Alpha integration, although also I stumbled across bugs as Parrott cautioned in the release notes. Weather correctly fetched my location, but Wolfram Alpha didn’t accept the (theoretically supported) “wa” command and some queries just didn’t work. And, obviously, being this a rough hack that’s not officially supported by Apple, memory consumption of the Flashlight plugin occasionally went through the roof with hundreds of MBs reported in Activity Monitor.