As a .NET developer, you must be familiar with CodePlex. There are some great projects there - I like to surf around and see what other developers are doing, and try some of the products. I've found a lot of things from CodePlex useful, so here are my top 3...

• BlogEngine.NET - which also happens to be the blogengine I used for this blog.
• Silverlight 1.0 JavaScript Intellisense - By default VS 2008 or VS 2005 haven't got JavaScript intellisense for Silverlight. This useful project gives you full JavaScript intellisense in your Silverlight 1.0 apps.
• patterns & practices - Enterprise Library - a managed class library for dealing with variuos scenarios such as caching, logging, database connection etc.

The patterns & practices team at Microsoft has several project on CodePlex. A lot of software factories - but the main one is the Enterprise Library which is a huge help, when you want something done without having to re-invent a lot of stuff... Also listen to this .NET Rocks podcast with David Hayden, explaining the Enterprise Library in more detail.