Many UML tool vendors offer good UML sites in addition to their tools. Here are some of the best:
Rational’s UML Resource Center: Lots of good material, especially some early stuff on UML (www.rational.com/uml).
Popkin’s UML Resource Center: Several papers and book recommendations (www.popkin.com/customers/customer_service_center/ enterprise_architecture_resource_center/uml.htm).