A slightly technical link:
TheServerSide.com - The State of The J2EE Application Server Market: History, important trends and predictions. Its a little outdated, but still quite relevant if you want a bird's eye view of what is happening in that space.
Among things that appear to have changed since the article has been written
- I am told that HP/Bluestone 8.0 is an awesome product, a huge improvement on the previous version AND open source to boot (though if the unlikely-to-happen marriage between HP and Compaq does happen, this product might be gone with the wind. I dont have much faith in Compaq's commitment to Java)
-IBM's Websphere is apparently playing catch up with BEA. A lot of bluechip clients are looking a lot more seriously at Websphere. IBM is also - cleverly branding their entire middlware product line (including MQSeries) Websphere. This would not only rub off some of the much deserved brand equity of those products onto Websphere -the application server, it may also allow them to claim a market share in advertisements that may not necessarily be that of application servers. That's a huge perceptual advantage. Between IBM and BEA, they may just redefine the application server /middleware space. Anyway, they are getting ready for a battle royale.
-iPlanet is trying to ride a 'Web services' wave. They certainly have a strong product line (Disclosure: My current employer is an iPlanet partner).
-Jboss - the freeware (?) product appears to be creating quite a buzz.

This post is probably very different in content from the others on this page. But I have committed myself to functional reading for the holidays. So, over the next fews days there would probably be more stuff in this vain.