At first, I didn't quite grok the concept of Ubiquity, but as I tried it out, it all made sense. Ubiquity has not only helped me be more efficient on the web, but it has cut down on the number of add-ons on I need to have installed. Examples of how I use Ubiquity include: defining words, translating in-page, mapping addresses, emailing pages to people, and looking up domain information.
The actual moment that led me to blog this was when I read that I can check my Google calendar using Ubiquity, regardless of what web page I am viewing. I've used Ubiquity to add events to my calendar, but I often ended up opening my calendar anyway, to check availability or to ensure that the event was added. I love that I can now check any date from my calendar; Ubiquity will not only show any events I have that day and the days following, but even displays them in all their pretty colors (each of my calendars is color coded). The best part is that the command does not need a specific numerical date, but can run off of conceptual date such as "tomorrow" or "next Saturday".
If you use Firefox, you should try out Ubiquity...if you are not using Firefox, why aren't you?