A website that I really started to get into a few years ago was http://tiles.ice.org ; unfortunately it seems to have died out. The closest I've been able to find was http://www.sito.org/synergy/ , but none of them have the same feel as tiles. Half the fun was not knowing what the other tiles looked like.

Does anyone know of any similar projects? Or just cool collaborative art projects online in general?

I had a good look for about 40 minutes for some similar sites Sunka, unfotunately what I found, were by no mean the same style or quality of the site you mentioned.

Sorry bud, I'll have another look when I have the time.