Up until recently, if I needed to do a quick unit conversion or sum, I'd have used Google Calculator. It has pretty basic features, but it's very convenient (on most browsers thesedays, Google Search is just up there ↗)
Now I have a new tool: WolframAlpha. You might have heard of it. During its recent release, a lot of journalists portrayed it as a search engine and said it was competing with Google. That's not quite true: it's a computation knowledge engine, and I'd say it competes with Google Calculator, not Google Search proper.
However, WolframAlpha isn't quite as handy as Google Calculator: you'd have to visit their site to use it. So, I created a Firefox keyword search from their main input field. The bookmark looked like this:
Ahh. Much better. I can now do searches for things like $17 + 10%.