The app I use daily is the official eBay app, but then I buy and sell on eBay, so it is just a must have for me! Works quite well too!
Also love the official USPS app, along with Package Buddy Pro.
Favorite reader app is Mantano. Also have the Nook, Kindle and Kobo apps for ease of buying books. Overdrive for library books.
Astrid task management is quickly becoming a favorite as well, likewise Evernote.