Some days ago I discovered ubiquity and I like it very much.
Because I use google define a lot, I wrote a very simple script for it.
You can find my script here: http://gist.github.com/45454 (I just changed the default google search script)
To use it, install the script, launch ubiquity and then just type gdefine [word], or type gdefine with a selected word and press enter. It's that simple :-)
I use this script all the time, so I hope it's useful for some of you! (there is already a define script but that uses http://www.answers.com but I don't like it as much as google define)
Other commands that I recommend: switching tabs (just type the name of the tab), tagging sites (tag [tag1 tag2 ...]), remember the milk commands (you can find it here), twitter