The authors of the scraping framework scrubyT have constructed an automated list of the top 10 Ruby and Rails blogs based on a combination of Technorati and Alexa rank.  (See the link for screenshots and commentary on each blog.)

10. weblog.jamisbuck.org by Jamis Buck.
9. weblog.rubyonrails.org by the Rails core team
8. slash7.com by Amy Hoy.
7. errtheblog.com by PJ Hyett and Chris Wanstrath
6. nubyonrails.com by Geoffrey Grosenbach
5. redhanded.hobix.com by _why the lucky stiff.
4. hivelogic.com by Dan Benjamin.
3. mephistoblog.com by Rick Olson and Justin Palmer
2. rubyinside.com by Peter Cooper.
1. loudthinking.com by David Heinemeier Hansson.

This post describes how the list was constructed using scrubyT (by aggregating a set of aggregators and hitting Alexa and Technorati for the rankings.)