There is quite a large number of free and commercial UML CASE and diagram drawing tools. Their (relatively extensive) list can be found here (see also the included section of external links). Selection of a favorite tool is up to the user's personal preferences, however, some of the tools are very poor in the implemented features and/or usability, but some exceed others. Follows a list of free tools that belongs to a set of better (maybe the best) and widely used UML tools nowadays:
WhiteStarUML - free UML CASE tool for Windows, a successor of well-known StarUML; very good support of UML 2 features, excellent extensibility, possibly worse usability for new users
Software Ideas Modeler - a good-looking free UML CASE tool for Windows; sufficient and relatively easy-to-use for usual modeling, not very good extensibility
Modelio - Eclipse-based UML CASE tool; works on Windows and Linux
yED - a free Java-based diagramming tool supporting also UML notation; not a CASE tool but can produce nice-looking diagrams, works on Windows, Linux and MAC OS X