SettingsProvider - gives you a type safe way to store application settings in a testable way (really cool IMO)
Async helpers - Execute.AsyncPattern/Execute.AsyncPatternWithResults will let you inline async pattern calls and a simple background worker.
var stuff = Execute.AsyncPatternWithResult(_webService.BeginGetStuff, "myarg", _webService.EndGetStuff);
new BackgroundWorker().DoWork(()=>{ /*somework*/ }, r=>{ if (r.Error != null) DoStuff(r.Result); }
Abstractions - the abstractions namespace contains interfaces for each of the main WP7 classes, like PhoneApplicationService/Frame. There are also adapters for real instances, and also Fake implementations in the wp7essentials.testing project
Testing - The testing project contains helpers which make unit testing for WP7 easier
Diagnostics - Trace adapter, memory usage classes
Think we should add something, let us know via the issue tracker!