I have always liked the Resin application container. I often use it to develop servlet and J2EE applications, even ones which are eventually deployed on another server. Resin is fast, clean, and easy to manage. Its cool ability to run PHP as well as java is a bonus.
Now it’s even cleverer, and it includes a bit more trendiness. Resin 4.0 supports a git repository as the underlying storage for web applications, with all the versioning and updating benefits which that implies.