I'm surprised no one has mentioned Code School. Their "Ruby Bits Part 2" course follows material similar to the second half of the excellent book "Eloquent Ruby.", with a nice set of interactive tests to help drill the material into you.
Link: http://www.codeschool.com/courses/ruby-bits-part-2
Levels:
- Blocks, Procs, & Lambdas
- Dynamic Classes & Methods
- Understanding Self
- Missing Methods
5 & 6. DSLs
It does cost $20 a month for a subscription (for which you also get access to all of their other courses), but for me it was totally worth it.