If you're a Twitter user, I recommend Tweetbot. It's a nice, clean UI and I think a leg up on the official Twitter app.
If you want to chat with people across multiple platforms for free, LiveProfile is a good one. It's available for iOS, Android, and BlackBerry.
Dropbox is a definite must-have. Nice to be able to access your files from wherever.
Coming from Android, you should pick up Google Voice. It's nice to have your voicemails transcribed and the ability to listen to them in any order. Unfortunately, the transcription isn't the greatest.
Finally, if you do Starbucks, you can't go wrong with the Starbucks app. You can set it up with your Starbucks card, find a store, refill your card and figure out what the real name is of your favorite drink.
Those are just a few of my favs and my $.02.
* I would have posted the links, but I don't have 10 posts yet.