I'm not sure if I've seen a side-by-side comparison of web frameworks including Seam. Seam is not really a web framework, it's an integration framework and/or application stack. Seam integrates JSF with EJB3 and builds/extends on functionality in JEE 5. And you don't have to use JSF for your presentation layer, you can use Wicket, GWT, Flex, or Tapestry.
The Seam application
stack includes the framework, the libraries, the
build script and project generator, the IDE integration,
a base test class, the Embedded JBoss
container, and integrations with many technologies.
Some of the main selling points of Seam are conversation support, jBPM integration, bijection, remoting, conversational web services, project generator (seam-gen), fix for LIEs via SMPC, PDF, Excel, Hibernate manual flush support, minimal xml
coding, drools support, Spring integration, and much more.
Best of all, the bits are free: Seam, Facelets, JSF Sun RI, RichFaces/ajax4jsf, Hibernate, JBoss AS
The best resources to do your research are the ref doc, SiA book and the Yuan book. You will find many similarities (and differences) between the Seam stack and the Spring stack, both of which have influenced each other to a great extent.