Are there any free tools to automatically create UML diagrams from an existing .Net/Visual Studio 2005 projects?
If you are using Visual Studio Team Suite 2005 (note: not 2008) I believe you have the option to reverse engineer the .Net code into UML in Visio, Note, this doesn't apply to Visual Studio Team Suite 2008!
To reverse engineer .Net code into UML with Team Suite 2005 you should be able to click on the following menu items: Projects -> Visio UML -> Reverse Engineer.
For Team Suite 2008 (just as an FYI, since you are using VS 2005), MSDN has this interesting link which has probably come from this MSDN Thread on the topic for Visio & VS 2008 which also lists some alternative UML modelling tools.
Also there's a list of free UML tools on this blog entry here they've listed the following:
The agilefactor DotNet2UML utility reads .NET assembly metadata using .NET reflection features and creates an XML representation of the resulting UML called XMI. This XMI can then be imported into most UML tools.
uml2svg is an XSLT-based tool for converting XMI-compliant UML Diagrams into SVG.