CollabRL is an attempt to make a multiplayer roguelike in which the players can edit the world around them. I didn't get as fancy edit controls as I would have liked, but you can at least walk around and 'drop' letters on the ground which other players see immediately.

Technically this was a very difficult project for me.. I had to learn threading (mutexes and condition variables yay) as well as proper asynchronous transfer with sockets. I also used sqlite as a database system for storing map information which had a bit of a learning curve, but ended up working very well.

Language used: C++
Libraries used:
libtcod --- for input and display
sqlite --- for storing map information
win-pthreads --- for thread trickery
zlib --- for network compression

Controls:
Arrow keys - move
Any letter - drop that letter on the ground for others to see