There are a handful of Source MODs that can be downloaded and installed straight from your Steam client. You need to own at least one Source engine game to be able to install these otherwise completely free games, and some of them specifically require that you own Half Life 2, but the requirements of each of the games are noted in their descriptions. Since Portal was recently given away for free in celebration of Steam for Mac, and you did get it – right? – there’s no reason not to check out these excellent games. The Naked Gamers picked five of their favorites to tell you about, and can honestly say that they’re worth your time and hard drive space. Check them all out after the break!