I have just clocked that the Enterprise Library has been released; you can download it here
I am quite new to using the Enterprise library and I don't use all of it but there are distinct advantages to using it when developing Enterprise (!) size applications. Especially where the Exception handling rules are concerned. For smaller projects I would probably consider the use of the library as overkill!!
Overview from the site
The patterns & practices Enterprise Library is a library of application blocks designed to assist developers with common enterprise development challenges. Application blocks are a type of guidance, provided as source code that can be used "as is," extended, or modified by developers to use on enterprise development projects. This release of Enterprise Library includes application blocks for Caching, Cryptography, Data Access, Exception Handling, Logging, Policy Injection, Security and Validation.
For those who are totally new to Patterns & Practices should definitely take a look at the MS home page. There is much more than application blocks to get stuck into!