Extracting Files
Archive Utility is an already installed program on Mac computers that is a good tool for extracting .zip files. However, the number of formats that it can extract are fairly limited, so I would recommend downloading Zipeg, which can extract several different compressed file formats, including .rar, which is a pretty common format that iArchive fails to open. When you're downloading your file, just set the program that will open your file to Zipeg, and it works like a charm.http://www.zipeg.com/
Archiving Files
Archive Utility also happens to compress files if you need to send something to another person or post your content to make it downloadable to people. However, another program that's useful to install is iArchiver.http://download.cnet.com/iArchiver/3000-2250_4-10774270.html?tag=mncol
Converting Files
If you need to convert a file from one format to another, there are lots of free options to do this. It's possible to download free programs to convert your files, but I prefer the two websites here because there's no need to download anything. Simply open the file that you want to convert, select what format you want it to convert to, and hit convert. In the case of Zimzar, you'll need to enter your email address so that they can send the file to you to download. Free File Convert takes you to a new page where you can just download your file to your computer, which completely bypasses the need to give your email address out. Either way, they're both reliable sources and they have a pretty good range of standard file formats, including image, document, audio, and video formats.http://www.zamzar.com/
http://www.freefileconvert.com/
