SharePoint Skinner

A great tool for customizing and branding a SharePoint Web Site or portal is SharePoint Skinner.
It is a desktop application which enables you to visual edit the css for your SharePoint and save it as a SharePoint Theme. It is very easy to use and very productive. You should read more about it on its tutorial here: http://www.elumenotion.com/Blog/Lists/Posts/Post.aspx?ID=4
You will find it on CodePlex too: http://www.codeplex.com/SP2007Skinner