For the past few years, the web server of choice for developers (but not necessarily for deployment) has been Mongrel. Mongrel was small, fast and didn't need much configuration, and it was blazing fast compared to the pure-Ruby Webrick server. What started out as a small project for a mixed C and Ruby server turned into a staple in the Ruby community. But software must change and grow, and Mongrel2 brings the project forward. Mongrel2 will keep the HTTP parser from Mongrel, but everything else will be new.
While the project is brand new (the first commit was on June 11th 2010 with the comment "Initial sorta working commit."), expect to be hearing more about this in the near future.