Does anyone know some good, simple online multiplayer games I can play with a friend through the internet?

I’m quite fond of Tetris Friends myself. The multiplayer’s a good time, but be warned! It get’s very addicting, very fast.

iSketch.net

MineCraft, if you set up a server or use one, and its really easy

Depends on what you define as simple, but Castle Crashers is loads of fun, irritating & mildly addictive, but definitely fun. I play on the Xbox where there are several DLC options on offer, it’s also available on the PS3.

1066 is fun to play with a friend.

Runescape :)