How to Create a Class Library in C# .NET http://www.homeandlearn.co.uk/csharp/csharp_s16p1.html
Walkthrough: Building and Deploying an ASP.NET 2.0 Virtual Earth Web Part to a SharePoint Site (databases in WSS!) http://msdn.microsoft.com/en-us/library/bb153523.aspx#WSS3ASPNET20WebParts_ChoosingBetween
Building Web Parts for Windows SharePoint Services 3.0 http://www.15seconds.com/Issue/060824.htm