Good PC Online Multiplayer/Co-Op with puzzles like Portal 2

[KiyoMoon]

We - that is a friend and I - enjoyed playing the heck out of Portal 2'S Co-Op mode, but after finishing both the "main story" and the "challenge mode", the only thing left to do where the community maps which can be quite hit-or-miss, so we are looking for Co-Op games, that require cooperation to solve puzzles. They don't have to be pure puzzle games, but it would be desirable if they contain a good amount of them.

So far the only entry that came to mind for my was the Trine series. But he hasn't said anything if that would be his cup of tea, so I am still looking for more ideas.

Free Games would be nice, but I know that they are rare, so paid Games are also good, as long as they are not too expensive.

 

[KiyoMoon]

Dunno if Ibb and Obb are up to his style, and Four Swords Adventures would only be possible via an Emulator, which can be a hassle to set up.

 

[KiyoMoon]

You could try Myst Online: URU Live if it is still available.
Good thing is that it's completely free, bad thing - it isn't finished. You can invite people to you worlds to help you with the puzzles as far as I remember.

I though about that too, I think I am gonna check if it's still online.

Lara Croft and the Guardian of Light/Temple of Osiris is probably a good choice too.

I checked them too, and asked his opinion on them, but he is a bit unsure.

Thank you so far, for your recommendations.