WSS 3.0 : How to add a new ContentType?
Here you go for the sample,
SPWeb MyWeb = new SPSite("https://<Server>/sites/Team%20Site/default.aspx").OpenWeb();
SPContentType DocTypes = MyWeb.AvailableContentTypes["Document"];
SPContentType MyType = new SPContentType(DocTypes, MyWeb.ContentTypes, "Test");
MyWeb.ContentTypes.Add(MyType);