Yesterday, 2 colleagues pointed me to two very useful tools for Visual Studio.
Mole is a data visualiser that has been tested on WPF, WCF, WF, WinForms and ASP.NET projects on VS2005 and VS2008, C# and VB.NET. Not only do you get a nice bit of software (for free!) but in addition, the creators have put together tutorial videos and a manual. You can download it here.
The other tool that was mentioned was PowerCommands for Visual Studio 2008 which seems to be an updated replacement for CoolCommands 3.0 (for Visual Studio 2005). Definitely worth a look.
Last but not least, I have not been keeping up with the latest developments in Mocking, namely Moq. This looks like an interesting library.
Thanks to John Barrett and Adrien Brown for the heads up.