Another nice online UML Diagram creator

can be found here and creates images like this:

UML Sequence Diagram Generation

fun to use and good results, at least for simple sequence diagrams:
http://www.websequencediagrams.com/

UML, quick and easy

If you happen to have the need to draw a few simple, good-looking UML diagrams, your choices are few. Either you turn to something commercial, overly bloated with things you currently don’t need, or you [...]