Personally, I prefer
Resin from caucho.com. Whether it is free or not depends on what you plan to use it for. Their "developer source" licence is free unless you or someone else is making money out of using it, in which case it is $500.
Another good, and completely free, choice is
Jetty. less well known than Tomcat, but used in an impressive array of products and services.
You may want to look at my
advice on choosing a servlet container page for more information.