I just installed an app on my phone which I am quite impressed by, so I thought to start a thread to share interesting apps.
http://merlin.allaboutbirds.org/
This one is available on Android and iPhone. It's from Cornell university, and it's for bird identification. I installed it because a friend is going to Guatemala for a holiday that will include some bird watching. Anyway, the app is small, but then you need to data packs for different regions that are between 100mb and 1gb. It's better for American (N & S) birds (includes range maps) but it still seems pretty good for the UK at a quick glance.
You can browse birds by type, but it's strength is bird identification. Either answer a very few question (where, when, how big, what colours, what was it doing) and it comes back with a range of images of candidate birds. I tried it on a Egyptian goose and it was the second bird it suggested. The other way is via taking or using an existing photo. I haven't tried this yet.
N.B. I am most distinctly not a birder/twitcher, so I can't say if it's a good app to the practices birdwatcher.
Edit: I tried the photo id and it works pretty well. I had taken a photo of some cormorants on a boat because - cormorants are cool. They were some distance away, so very small. I zoomed into the most distinctive profile, which was highly pixelated but .....tada! It got it in one.