Blueprint Software Modeler Community Edition is a free UML2.1 modeling tool for non-commercial users. It supports the most important UML diagrams:
Class Diagram
Package Diagram
Component Diagram
Use Case Diagram
Activity Diagram
State Machine Diagram
Sequence Diagram

Additional diagrams currently under development, making the UML modeling complete, will be included shortly:
Composite Structure Diagram
Object Diagram
Deployment Diagram
Communication Diagram
Interaction Overview Diagram
Timing Diagram

Additional features on top of the UML diagrams:
Integrated with and built on top of the Eclipse3.2 IDE
Automatically validates your design through its validation engine which checks all UML2.1 constructs
Complete implementation of the UML2.1 specification; rich semantics of all UML2.1 elements and features, reflected in the diagrams

General features:
Extended Documentation on all UML constructs; changes in the Model are automatically reflected in the documentation.
Detailed requirements whereby the quality (sub)characteristics are described according the ISO 9126 Standard
Copy & Paste
Drag & Drop
Printing