Integrated Development Environments (IDE) and Design Tools

Dr. Keuneke's elements of design page specifies requirements for labs including ADTs and object models using UML.

The following are some open-source, free tools:

http://argouml.tigris.org/
http://www.gentleware.com/products/poseidonCE.php
http://www.eclipse.org/ and a tool to make OM's through Eclipse with an example how it works
http://www.fujaba.de/
http://www.omondo.com/index.jsp (needs the free Eclipse IDE)
http://objectclub.esm.co.jp/Jude/jude-e.html
http://uml.sourceforge.net/index.php (requries KDE desktop for Linux)
http://www.htc.honeywell.com/dome/ (written in Smalltalk)
http://www.essmodel.com/index.html (UML reverse engineer)
http://www.objecteering.com/products.php (free personal edition available with no round-trip engineering)

The three leading contenders seem to be the first three: ArgoUML (open source), Poseiden Community Edition (closed source, extends ArgoUML), and Eclipse.

I think these often have student versions that are free as well