The folks at Plumbr have done some interesting data harvesting from the anonymous phone home data provided by the free version of their memory leak detection system.  This has allowed them to determine the most popular application servers from their user base.
From over a 1000 installations they were  able to to inspect the classpath in order to look for an application server and then to plot the results they found:
So Tomcat is the expected market leader with 43%, but Jetty comes in a very respectable second with 23% beating Jboss with 16%.  Now this is not a hugely scientific study and the results are from a self selected sample of those that are concerned with memory foot print (and hence might be more favourable towards Jetty),  but it’s still great to see us up there!