I am in the process of generating a more complete, new set of benchmarks that includes a wider variety of contenders. You can find the original set of benchmarks at http://swiftiply.swiftcore.org/oldbenchmarks.html.
There was a discussion on irc.freenode.net, in the #merb channel, last night (June 10th), where perlbal received high praise. So this morning I grabbed it to do some quick and dirty benchmarks. All of the numbers show an average from 3 runs of ab, issuing 20000 requests each time, with one discarded warmup run. Note that this is just some quick and dirty teaser numbers. The full stats report will contain more thorough analysis, including standard deviations on the run results.
The instance of Perlbal used was configured to only handle a single port, no virtual hosts, and is doing nothing except serving static files; there was no logging of files delivered. RAM usage, as reported by ps, is (VSZ/RSS): 87548/14352
The instance of Swiftiply used is the one in production on this server, handling this site's traffic plus about 28 others; every file delivered IS logged. RAM usage, as reported by ps, is (VSZ/RSS): 57868/24916