The followings are some iOS API that I found useful.
- fmdb. A Cocoa / Objective-C wrapper around SQLite Instead of use sqlite C interface, use this API to make your life easier.
- MBProgressHUD. A modal progress view. You can use it to tell that users login/download/loading is progressing
- ASDepthModal. A modal popover view container Use it to popover a view and get the nice blur effect.
- ASIHTTPRequest. A nice http and network API This is a nice alternative for NSURLRequest. ANd it comes with a good tutorial.
- cocoacontrols. A website where you can find tons of open-source iOS UIs