For start with the language:
- Ruby Documentation
- Why's Poignant Guide To Ruby (offline since August 2009)
- Mirror 1 (Scribd)
- Mirror 2 (PDF Version)
- Mirror 3 (HTML Version)
- Programming Ruby
- The Book Of Ruby
- Ruby online interpreter
And if you'll use Rails, these screencasts are excellent:
- Ruby on Rails Screencasts
- Railscasts.com
