Access to tested generators, a designer for building your project,
FXCOP'd source files, XHTML,
NUnit Testing Code, VS.NET 2005/Express solution files and SQL scripts.
xacc is an opensource multi-language IDE written in C# ... it handles syntax highlighting etc for everything from Boo and C# to Ruby and Perl, to Caml, F#, Scheme and Lua, and even Yacc, Bison, Lex and Flex ... and PowerShell.
It has project support for C#/C++, Nemerle and Boo, and Yacc/Flex, NSIS, etc.
An open source project which allows you to create help files (.chm and .hxs) from pages on the MSDN library (presumably, you could convert this to create them from any html pages?)
The new NetBeans Ruby plugin sounds relly impressive: "features that go beyond basic editing, syntax highlighting,
navigation outline, project support and unit test execution" to include extensive code completion, integrated documentation tooltips and even semantic analysis and highlighting!
A job scheduler like quartz.net (or like Java Quartz) to offer enhanced scheduled-tasks (or cron) -like functionality to .Net applications at the thread/process level.
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.