Although I haven't used any of them yet, the various beeware projects have always been interesting to me. I've especially been watching Batavia [1] because I think it could be a very interesting way to ship form/page validation code written in python to web clients. Lots of projects have tried this in the past but I think the approach it is taking is interesting in that the resulting code is relatively small (compared to other approaches) to serve.

http://pybee.org/project/projects/bridges/batavia/