I might be biased being on the Trinidad team, but I still think it's top ... esp. as it handles high concurrency well while still being much lighter than TorqueBox (which I would also recommend if you do want something beyond just a web server e.g. built-in jobs but be aware that some of it is TB/JRuby specific). Most other options mentioned are Java web servers, which you can rule out if you do not want to install a Java server, warbling your application and deploying it as a .war file.
p.s. for the (next) version 1.5.0 re-deploys are to be revisited and one should be able to do some (memory leak-free) zero-down time deployments.