Since apps are generally inexpensive and easy to try out I'm just going to make a list of my favorites as a starting point. You can check them out and judge pretty quickly whether or not you want to try them. PS Photoshop is the most expensive ($10) but it's worth it if you want to do serious photo editing.
I have only used these apps on the iPhone and iPad. I can't be sure they are available on Android or Windows but many probably are. (Note: I'm not a big time gamer - not even a small time gamer!)
Storm, WunderStation, Work Time HD
ImageChef+, Panorama(DMD), Photo Delight, PS Touch (Photoshop lite), FrameMagic, SnapStill
Keynote (presentations), Pages (word processing), EasyCalendar, Microsoft OneNote, Calculator(MyScript), Dropbox
IMDb (movie database), TV Guide, Pandora