1. Facebook - Just because it's Facebook. I use Friendly on my iPad which I prefer to Facebook.
2. Twitter - This is pretty much my primary news source anymore.
3. iBank - I use it to enter any expenditures throughout the day. Syncing it with the Mac iBank application makes balancing my bank account a lot easier.
4. iCatcher - My podcast app. It syncs across multiple devices.
5. Todo - Just because it seems like I always have a lot to do. It's pretty flexible in that it allows you to maintain multiple lists, tags, etc.