I have an iPhone, but the apps should work the same (though there may be some concern re: image quality on the larger screen)...
The two apps I use the most (and would recommend) are:
iONTBIRDS - Free app, lets you check the ONTBIRDS list, for updates. It checks posts against known locations, and will plot the location on a map, iff possible.
iBird Explorer Plus - Can't recal price, but guess it was around $20 (covers North America, but there's a cheaper "Canadian only" version as well). It's an interactive guide, with hundreds of birds listed, includes pictures, information and recordings. You can search by attributes (e.g., size, colour, location seen, etc); basically, it's like having WhatBird.com with you, all the time. Once you've looked up a bird, you can easily add birds to a favourites/checklist. I also like the fact that the app is updated fairly regularly - adding more content, updated pictures, etc.
