[iAinsley]

Use AlfredApp for arithmetic ;)

[lloeki]

The fact that Spotlight does arithmetic seems like it's regularly overlooked.

[gee_totes]

or the command line utility bc

[lloeki]

If you use bash, you can also use $(( 3 + 2 )). More often than not, if it gets cumbersome to fit it into Spotlight, I'm going to fire up a Python console.