MacOSX has it's own built in Zipper & unzipper with various names depending on which OSX version you are using. In Snow Leopard it's simply called Archive Utility.
Select the zip file in finder, go to file menu & choose Open With & at top should be the OSX archive utility.

But UKBobboy01 is right, you do need to be careful with zip files. Though OSX is unaffected by virus.

Not very commonly but once in a blue moon you find mac files that are RAR. Or it might be a music or other type of file that has been archived into a RAR file. For those I recommend UnRarX which is a free utility for OSX.
(there seems to be a unrarx 2.2 (interface version) & 3.2 (command line version) possibly created by different people, one originally windows. I use the 2.2 version & gave you that url instead.)