Here's a short list of COMET solution I tried, with pros and cons:
Python Twisted: Non-blocking server based on Python. Unfortunately, "eats" a lot of CPU and doesn't scale very good;
Jetty: Very good, if you doesn't need to serve more than 10k clients simultaneously. Jetty consumes ~2GB memory per 10k active users;
Apache Tomcat: The same problems as with Jetty - eats lots of memory;
Apache Mina: NIO framework (non-blocking IO). Is not documented very well, and has problems with scaling;
JBoss Netty: NIO framework based on Apache Mina. Has a weak documentation, as well, but shows the best performance compared to aforementioned solutions. With Netty you can serve ~100k connections at a time, consuming several gigabytes of memory and using ~20% CPU (4-core);