It’s an alpha Firefox plugin that’s attempting to be a command line for the internet. It reminds me a little of yubnub, but quite a bit more powerful as it’s available on every page and is context sensitive.
Essentially, it has a set of built in commands (that you can add to an extend) and it’s aware of the current browser context, so if you have something highlighted, it can act on that subset of the current page.
Previously, I’ve been a heavy user of Firefox smart keywords, which allow you to assign aliases to bookmarks and type the aliases in the location bar. I’ve created smart keywords that allow me to search wikipedia, amazon, imdb and the dictionary. Ubiquity has all of these, plus a lot more built-in.
I’ve only been using it for a little bit, and there are some rough edges, but I think that there is quite a bit of promise here as well and thought that there might be a few other keyboard jockeys out there that would appreciate what Ubiquity is trying to do.