SubdomainFu is a Rails plugin to provide all of the basic functionality necessary to handle multiple subdomain applications (such as Basecamp-esque subdomain accounts and more).

A rails plugin that handles subdomain accounts

Set of protected methods that use the subdomain as a way of identifying the current scope

Rails extension to help with cookies in a system with sub-domains

SubdomainRoutes add subdomain conditions to the Rails routing system. Routes may be restricted to one or many specified subdomains. An URL will be recognised only if the host subdomain matches the subdomain specified in the route. Route generation is also enhanced, so that the subdomain of a generated URL (or path) will be changed if the requested route has a different subdomain to that of the current request. Model-based subdomain routes can also be defined.