Steam is a game client for Macs and PCs
the downside is if you dont wnat to spend money dont get steam
if you have $30 around get steam then get the orange box
after you have those you can get tons of free mods
Theres reasons there are a lot of hackers in free games
a. they're free
b. because the developer isnt making money they are less likely to:
-come out with updates/patches to stop hackers
-care about hackers
-Moderate theyre servers
On a side note a good free game for Steam is Fortress Forever which is basically a free version of TF2
very few ppl play it though