What are the best free online "English as a Second Language" (ESL) sites?

I used to teach English as a Second Language in Europe, but that was over ten years ago. Now that I'm basically clueless regarding what is useful online, I would like to help a couple of young Slovenian friends to improve their English skills online without them having to pay anything. They are attending universities there, and already have a workable knowledge of the language but would like to improve their understanding through repetition and practice without taking away too much time from their other studies.

I would prefer school/tutorial, grammar/usage sites with lessons more than penpal-type sites; there are several English-speaking friends with whom they can already visit to practice. Some formal lessons would offer them some useful examples and drills that can be beneficial to them..

Can anyone help me help them by suggesting the best site(s)? Your assistance would be very much appreciated on both sides of the pond!