My favorite so far is the Remember the Milk / Gmail mashup. I’d never even heard of Remember the Milk, but this has become my new favorite thing.
Worth noting in the screenshot:
Remember the Milk To Do List
Multi-colored, “faviconized” tabs!
I don’t actually use the “faviconizing” feature in day-to-day life because I like being able to see the page titles (I just used it for the screenshot), but it’s great when you have eight zillion tabs open at once (which is a regular occurrence for me).
No spam, invite box, or ads in my Gmail!
I also downloaded Better GReader (hugely helpful extension for Google Reader) and a slew of others. I definitely recommend scanning this list for time savers that will work for you!