The JavaScript library I use most after prototype is YUI so a big thanks to Chetan Patil for making it much easier to use in Rails.
Ruby Inside has a post on Ebb a small and fast web server for hosting Rails and Merb applications (and soon Django).
Ebb is a small, extremely high performance Web / HTTP server designed specifically for hosting applications built upon Web frameworks such as Rails and Merb (and, in future, apps on other non-Ruby frameworks.) The design is event based (similar to that used by Ruby daemons that use EventMachine) but Ebb itself is written in C and dispatches requests to Rack adapters. This is a real leapfrog over the popular Mongrel and Thin daemons which are primarily written in Ruby, and results in scary levels of performance.