If you're running Ubuntu with Unity, this Lens will put Ask Ubuntu right next to your files and applications in the Unity Dash. Via filters, you can select any other Stack Exchange site and find anything you need right there.
The app is licensed under the GPL version 3 and it's gratis.
To install it, jump into the Ubuntu Software Centre and search for "Ask ubuntu", then hit 'install'.
This works on Ubuntu 11.10 - Oneiric Ocelot.
The code was written by Stefano Palazzo and Mark Tully, with substantial help from Ken VanDine, David Callé, George Edison, Jorge Castro, Andrew Mitchell, James Gifford, and David Green.
For Bugs, Questions, Feedback, and the source code, see The Ask Ubuntu Lens at Launchpad.
You can also leave feedback and ratings in the Ubuntu Software Centre.
The client code is written in Python 2, using the Unity Lenses API, gir and dbus. You can get the code at launchpad. Two independently developed search engines, written by George Edison (available here) and me, power this app in "the cloud", but you can hack the app without knowing about them, as it will fall back to Stack Exchange API search.