Slavehack

Slave hack is a free online web-browser hacking game. It's been around since 2005, and it will stay free forever. You will hack into static non-player websites to get yourself started, but as you progress you will also hack and be hacked by other players. You will try to earn cash infecting computers with viruses, to upgrade your computer, pay for IP resets, purchasing web domains and purchase websites to store your files and source code. This multiplayer game is a lot of fun and is definitely worth a try if you're into hacking.