Here are some sample UML diagrams created with UMLet, with screenshots and UMLet-uxf-files (a simple XML-file format).
Please notify firstname.lastname@example.org if you find any syntax errors.
You are invited to send us further sample diagrams (or even UML poems); we'd gladly publish them on this site. Send the UMLet-file, along with both a large and a small screenshot to email@example.com. To create the large screenshot just use the "Save as JPEG" option in UMLet; to create the small one, shrink the image to 20% of its original size.
UMLet UML Tutorial: