Right now things are in a state of flux, with some vendors lagging behind the current API specifications. The best advice is to make sure it runs on the "Reference Implementation" - ie, Tomcat. Bill
I'm not sure if this was actually the question you wanted answered, but most available web servers do not support servlets as supplied. There are servlet extensions available for most of the big name servers (Apache, IIS, iPlanet ...), but they all need to be installed by a system administrator which may not be fasible if you are just renting web space on a public host. If you have control over the software and configuration of your web server, then you can choose a plug-in servlet extension which works with several web servers to be assured of at least basic compatibility. Resin, JRun and ServletExec seem fairly flexible about partnering with other web servers, others may be more specifically tailored to one server.