The DataMapper is an Object-Relational Mapper for Ruby. It integrates with Rails or can be used stand-alone.
Speed. Speed. Speed. The DataMapper is fast. On average, between 2 to 5 times faster than ActiveRecord under most conditions.
Easy on hardware. DataMapper uses less RAM than ActiveRecord, both to load the library itself, and during operation.
Easy on the mind. DataMapper's philosophy is that you shouldn't have to work for performance. You don't have to "include" associations to eager-load. Finders are ActiveRecord compatible, but they also allow for an even simpler, more terse Enumerable-like syntax.