Open up Automator and make a new service. In the top bar, set Service Receives to URLs and In to Any Application. Drag over the Run AppleScript action (from the Utilities section) and paste the following code into its entry window, replacing anything that was already there:
on run {input, parameters}
return (do shell script "/usr/bin/curl 'http://qgf.in/api.php?url=" & (item 1 of the input) & "'")
end run
Below that, drag the Copy to Clipboard action. Save the Service as Shorten URL or whatever else you want.
That done, you should be able to Control-click on any URL in any program and select Shorten URL from the Services submenu; the shortened URL should then be available in your clipboard, ready for pasting into Twitter, e-mail, or wherever you need it.
This hint uses the lesser-known shortening service QGF.in. You can use other services too, as long as they have APIs. To use another service, replace http://qgf.in/api.php?url= in the script above; the syntax is usually http://shortener.name/api?url=.