if you're asking about a JavaEE app server, JBoss is a safe bet.
If you're asking about a ASP, then I have no clue, last time I checked, most don't offer Java hosting...
or if you're asking about a clustered environment, refer to JBoss in Action or other clustering guides.
They've been my host for a few years. FAST and reliable. All linux machines... You can go cheap and small with your own private JVM or go nuts and get your own server.
Tell them I sent ya, if you are interested.
I wonder if anyone has experience using Seam with EC2?