Yesterday I had a chance to tinker around with a new project using Merb, DataMapper, RSpec, and Thin. Merb + Thin means your app serves up pages twice as fast, with half the memory. Merb + DataMapper means your app is threadsafe. RSpec + DataMapper means your tests are completely independent of the database - which means no db:test:prepare step, so the tests run lightning fast and without fragile database dependencies.
Above all, these components are all cleaner, smaller, and more specialized than their mainstream counterparts (Rails, ActiveRecord, Test::Unit, and Mongrel). It’s too early for me to make any kind of real judgement about these tools (except for RSpec, which is very mature and entirely suitable for production use.) But using these four tools in a stack together leaves me with the gut sense that this is where the future lies.