the idea belongs to http://tryruby.hobix.com :),
but this system is in python and firstly aimed for Python (2.x currently), but actually any programming language can be taught.
as real interpreter for expressions is not really needed -- you can just provide expected input.

Beta online demo http://84.55.3.131:801/ (loads very very slow :/ my home router issue, maybe I'll implement ajax soon)

For local Python learning it's recommended you use start_IDLE.sh (for linux) or start_IDLE.bat (for windows), as then you'll get all the nice features of IDLE (syntax highlighting, available funcion list for objects, autocompletion)

People not familiar with bazaar/launchpad can download the zip package
http://files.akl.lt/users/jurgis/python/intro-to-code.zip (old version -- no web2py integration)