Sun take open source seriously these days, and users seem to be starting to take Sun open source seriously too.
GlassFish was late to the party in open source application servers, but it seems to be gaining traction. And, more importantly, it actually seems to be pretty good. Various Interface21ers, including Costin and Juergen, have taken a look at GlassFish and given it the thumbs up (although we haven’t yet worked with it in production). From what I’ve heard, performance is excellent–probably substantially due to the reworked servlet engine based on NIO. The JPA implementation–TopLink Essentials–should perform well too, being based on the mature and performant TopLink engine.