For those that might be taking classes that require some Use-case Models or similar diagrams, UML systems can be tough to find that don’t require a $1,000+ license. I did find a program called Visual Paradigm for UML CE (Community Edition) that doesn’t require payment for some basic functionality. While it’s got some features locked out that would have been nice to generate some files for classes, the UML portion works pretty well and (on Mac, at least) allows output to PDF. There isn’t a cost, although you do have to register to get an activation code. It’s Java-based, so should work on Windows, Mac (that’s what I’m using) and Linux.