I've tried quite a few and still prefer Resin for most uses. It's standards-compliant, fast, easy to install and configure and free for development.
Tomcat is the official reference platform, but it can be clumsy and slow, and tends to need a lot of restarting.
Jetty is free, standards-compliant and very configurable. Jetty is a great choice for embedded servers or other resource-constrained uses.
BajieServer is also free and pretty compliant (with API 2.3 at least) and great for
testing as it doesn't expand war files into a directory but serves them direct from the war.
GNU paperclips is a supposedly ongoing free servlet container project, but it can be hard to find a usable download.
Any more suggestions from anyone ?