Grigori just posted about Enterprise Library being downloaded 2.000.000 times. As you may know I am a big fan of EntLib as it is a great way to reuse plumbing code and focus on business functionality. EntLib 4.1 is the most recent version, ready to use with Visual Studio 2008 SP1. EntLib 5.0 is under construction. Read all about it here.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.