StuffIt is the most common file compression utility on Macs. Like similar applications, you can compress and uncompress files and folders. If your transferring a large number of files by FTP or e-mail, the best thing to do is compress them into one file for an easy upload.
There is a free version of StuffIt, but recent versions only contain the uncompression utility. It no longer included DropStuff (the compression application). So, in order to create .sit files, you need to purchase the commercial version of the program which isn’t that expensive.
What do you do if you don’t want to buy the application? OS X has a built-in ZIP compression and uncompression tools that very few Mac users seem to know about. I think it pays to read manuals, but most people don’t read these days and just start using their high tech toys out of the box.
Compressing a folder is very simple. All that you have to do is browse to the folder, Control-Click on it, and you’ll see a context menu. Select Compress "folder name" and it will create a .zip file. The contents of that folder will then be in one easy to manage file.
Be careful using 3rd party ZIP utilities. I’ve noticed on some that they don’t handle resource forks very well and save Adobe Type 1 fonts as zero byte files. Also, be careful uncompressing Mac archives on a PC. Even if Type 1 fonts are correctly stored in the archive, they will lose their resource forks in Windows and will save as zero byte files.
The .zip utilities built into Mac are ideal for transferring data to another Mac and they’re free. Since OS X is built on BSD, you also have the option of using gzip and tar compression, but you may lose resource forks using them so I would not recommend that approach. Stick with the built-in zip tools on the desktop.