A list of some resources for learning Ruby (not Rails) and my comments about them. I'll add to and edit these as I go. Ruby is the important part, after all.
Learn Ruby the Hard Way You start with the very basics, working in irb, not in their own online environment. I like it.
Rubymonk One of the many Bhuddism-flavored learning resources that is a fun way to practice your skills and pass to the next exercise--and maybe pick up a little wisdom along the way. Browser environment.
Ruby Koans My first exposure to Ruby, and one of the tools that made me love it. There is an introduction to testing methodology as a way to solve the koans.
Project Euler Not specifically Ruby, but this is a series of algorithm exercises that you can work out in using Ruby. Break it down!