A while back, Jason Langridge posted a table that compares the different versions of Windows Mobile 6 (Classic, Standard and Professional). He has now followed that up with an update that includes a side-by-side comparison of Windows Mobile 6 to its predecessor, Windows Mobile 5.0. The document also shows what Windows Mobile 6 features require Exchange 2007.
Definitely a great resource that puts a lot of information only a glance away.