Anywhere you can type text, you can type an AppleScript statement or two , select it, and use that Services menu item. The selection will be replaced with the results of the script.
I often leave Script Editor running in the background, so it's like having a handy calculator. If you're using a browser with Services support, try it on these:
133 * 1024 / 2.5
1560 as feet as meters
date string of (current date)