Since most of my web surfing consists of searching Google, the Google Toolbar saves me a huge amount of time each day. Now the 2.0 version saves me even more time by blocking pop-up windows. Thank you Google. Download it here: http://toolbar.google.com
Here are some more free libraries for .NET. I've been meaning to get all these links posted for a long time.
DotNetWidgets controls: Highly recommended. CropMonkey uses the included toolbar control. The VS designer support for these controls is excellent.
HTML Editor component for Windows Forms.
Utility library with VS-like toolbar control.
XML-RPC.NET: Great library for interfacing with XML-RPC services from .NET.
Mentalis.org has a great collection of open source .NET code. They have written C# class libraries wrapping the Windows data protection and cryptography APIs, a packet sniffer, a fully-functional proxy server, an SSL/TLS socket implementation, and much much more. High quality, reusable code, too.
I found this site that has some code and several samples for creating VS add-ins in managed code. Looks pretty interesting. The main sample is an add-in for Lutz Roeder's .NET Reflector.
Here's a page that explains how to get Visual Studio designer support for web forms in Class Library projects. This might be a good way to get around Visual Studio's annoying inability to open a web project that isn't already available through IIS.
O'Reilly has made available an appendix on String.Format() from C# in a Nutshell. Makes a great reference.
I saw this interesting analysis of the Sun vs. IBM clash over Java on Eric Sink's weblog.