1) MapQuest map application. The best thing I like about it is that it has voice directions in addition to the standard list and follow-feature on the map. It also has local business listings but its not that strong with regards to information, reviews, etc.
2) Yelp for local business information and reviews.
3) Pandora for streaming radio. You'll have to invest some time on Pandora so it can get better at creating your playlists. I also find it better for niche categories rather than playing the current Top 10 songs.
4) Mashable for keeping up with the latest in social media tech news. The app has a great user interface that makes it easy to digest the latest posts.
5) Dictionary.com for a great dictionary application. I love the fact that it also has the 'pronounciation' feature.
6) If you're into Fashion, Style.com's app is a great, quick way to check out the latest in shows.
I travel alot, driviing mostly, and I have become a weather nut. The Weatherbug Elite is the greatest weather app I have ever had and I have had a quite a few. If you don't already have Kindle, and love to read, you are really missing out. The Pandora Radio is pretty cool and free, also, but I haven't used it as much yet.
For me, the single greatest app for the iPad is Flipboard. Makes consuming Twitter and FB feeds, as well as a ton of other news and info sources (e.g. Wired magazine) easy and beautiful. Basically, Flipboard pulls in the info from all the links your friends post online, and then organizes them into a magazine. I can't praise this app enough. It's beautiful, fast, and very helpful.