A few iOS Mobile UI Patterns Galleries have popped up over the past couple of years. They do a pretty good job at putting together A set of screenshots per objective, like launch screens, navigation bars, etc.

Here are 3 that I've run into and have heard many others talk about. Note that none of these currently have an iPad galleries. For iPad, I recommend checking out ThoughtBot's great site, LandingPad

• Pattrns
• Mobile UI Patterns
• LovelyUI

I also haven't found one for Android and this is the best I've found so far Android UI Patterns.

Please feel free to contribute in the comments and I shall add them to the list.