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.
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.
Bike is a Web application framework that can make feature-rich applications using HTML files only. You need no database setup (by default), no scheme definition, and no command-line voodoo. Just put a good old HTML file under skin/, and your new app is already running. Bike is front-ended by Rack, back-ended by Sequel.
RackMap is a Web-based datacenter management tool. It was designed to help organisations catalog and manage datacenter assets such as racks, servers, routers, and switches. It allows you to catalog hardware location and cabling information, export data in various formats including PDF, generate bulk configuration scripts, and remotely control some devices.