Google Voice Saves me on text packages, on long distance (I can call my family in Canada for free!), and from listening to my voicemails at all. Can't beat it.
Google Goggles This is the app I whip out to demonstrate to naysayers just how cool a) smartphones and b) Android-based smartphones are. It's fun and useful and always makes me feel like we've entered The Future.
Gmail I have my personal and my work (Google apps) email on this in a wonderfully integrated and intuitive way.
Google Translate Remember Babelfish? Neither do I. The audio input and recognition and translation are astounding. Now if only it worked offline...
Maps I am lost. Always. This is the only reason no one else knows. Saves my butt whether I'm walking, driving, or taking public transit.
Google Reader A new offering from Google, and one long-overdue, it still has a few kinks to be worked out, but not many. Read and share the contents of your Google Reader, free!