quote: "Mono.Options is a beautiful command line parsing library. It is small, succinct, a joy to use, easy and powerful, all in one." ... well, I don't know about that, but it's simple enough :)
ZedGraph is an OpenSource .Net graphing library (2D graphs and charts) for Windows -- it can render multiple graphs in a single pane and offers extensive customizability.
Although it's nice to save the window position and size automatically ... it's frankly not worth an external library. However, this is a good sample of how to do attached properties, and if you have a general utility library that you include in all your WPF apps, this would deffinitely be a useful addition to it.
PostSharp Laos is a post-compiler, MSIL injecting, high level aspect oriented programming weaver. It looks amazing, and has been integrated with the .Net Enterprise Library 3.0 via the Enterprise Library Contribution project.
7zSharp is a .NET 2.0 LGPL wrapper around the 7z LZMA SDK and executable written in C#, providing a library (DLL) wrapper and simplified API to encode and decode using the 7z library.
NodeXL (formerly known as NetMap). An Excel 2007 template for viewing and analyzing network graphs, with a set of .NET Framework 3.5 class libraries that can be used to add network graphs to custom applications.