I've been accumulating resources for code katas recently so I'm posting them here in the hope that I don't just forget them or lose them down the back of the sofa.

• TDD Problems
• CyberDojo by Jon Jagger
• CodeKatas.Org screencasts of developers performing a kata.
• Ruby Quiz
• Ruby Koans by the late Jim Weirich and Joe O'Brien
• Coding Dojo by Emmanuel Gaillot
• Software Craftsmanship Code Katas
• Kata Club by Nola Stowe
• Code Kata by Dave Thomas
• MicroBlogging a nice multi-step project to interact with Twitter in Ruby. More of these kinds of tutorials are available at JumpstartLab's tutorials.

Although there are a few in this list that have language specific solutions, most of them can be attempted in any language.

The last site in the list also has a very nice article about Kata and the Dreyfuss Model of Skills Acquisition by Dave Thomas that is well worth reading.