Rainbows! is an HTTP server for sleepy Rack applications. It is based on Unicorn, but designed to handle applications that expect long request/response times and/or slow clients. For Rack applications not heavily bound by slow external network dependencies, consider Unicorn instead as it simpler and easier to debug.
Unicorn is a Unix and LAN/localhost-optimized fork of the Mongrel HTTP server. It is takes full advantage of functionality exclusive to Unix-like operating systems. It will reap and restart workers that die from broken apps, and there is no need to manage multiple processes yourself. Load balancing is done entirely by the operating system kernel. Requests never pile up behind a busy worker. The server does not care if your application is thread-safe or not, as workers all run within their own isolated address space and only serve one client at a time. All Rack applications are supported along with pre-Rack versions of Ruby on Rails via a Rack wrapper. It also supports atomic log cycling, nginx-style binary re-execution without losing connections, before_fork and after_fork hooks, and optional copy-on-write-friendly memory management.
Zbatery is an HTTP server for Rack applications on systems that either do not support fork(), or have no memory (nor need) to run the master/worker model. It is based on Rainbows! (which is based on Unicorn (which is based on Mongrel)) and inherits parts of each. Zbatery supports your choice of all the thread/fiber/event/actor-based concurrency models and Rack middleware that Rainbows! supports (or will ever support) in a single process. Zbatery will still exploit certain features of Unix for transparent upgrades, log reopening, and graceful stops, but does not rely on them for basic functionality.