The Apache Tomcat web server and Servlet container has been significantly upgraded in version 7 to now support Servlet 3 spec and many other changes.
For info, check out this video recording of a slideshow presentation, by Mark Thomas, the Tomcat release manager.
An article about the new async processing feature in Servlet 3.
An Introduction to Servlet 3.
The Servlet 3.0 spec (JSR 315).
Jason Hunter seems to no longer be writing his famous “New Features in Servlet X.X” articles. But to see the evolution of Servlet technology, you may want to read his past articles for Servlet 2.5, 2.4, 2.3, 2.2, 2.1. I believe Mr. Hunter was also the author of these Servlet Best Practices, parts 1, 2, and 3. And his article on Servlet Filters.
If you are new to Servlet technology, start with this Wikipedia page.