Aside from the built in stuff like email, calendar, contacts and maps with the traffic view I use the following every day: shuffle as my to do list, and StreamF

Aside from the built in stuff like email, calendar, contacts and maps with the traffic view I use the following every day: shuffle as my to do list, and StreamFurious (free version) for streaming audio. The TripIt and Yelp apps were useful on my recent vacation.