After getting frustrated with GNOME-Do's memory hogging, I switched to Kupfer. Kupfer is a lightweight, extensible application launcher like Do (as it is now called) but much more powerful and easier to extend (it is written in python).
So far, I've written the following plugins:
gwibber_plugin.py: This plugin allows you to send messages from Kupfer through Gwibber (with no configuration).
exaile_plugin.py: This plugin allows you to pause, play, skip, and go backwards in Exaile. It is based on the Rhytmbox plugin.
+1: evolution_plugin.py: I did not write this plugin (although I did do a fair bit of editing). The Evolution plugin adds an evolution contact source (and works with the built in email plugin).