iOS: The popular font-identifying website WhatTheFont now has an iOS app, letting you quickly identify fonts you see outside of your computer screen.
As far as web tools that ought to have mobile apps go, WhatTheFont really leads the pack. The iPhone's (or iPod touch's) camera really makes a great companion. If you're ever walking around and see a font you really like, you can snap a picture with the WhatTheFont app and upload it directly from your phone. Just like on the website, it'll work with you to identify the individual characters and then make font suggestions. While WhatTheFont isn't always perfect, when you get a clear picture it can get pretty close. It's just one of those things that can be extremely handy to have in your pocket if you're always on the lookout for great new typefaces.
WhatTheFont is free and available now in the iTunes App Store.