7-Zip

Original Name: 7-zip
Webpage: http://www.7-zip.org/
Latest Version: 4.65
Latest Beta Version: 9.07b
Available for MAC?: Yes. p7zip, EZ 7z.
Available for GNU/Linux?: Yes. p7zip.

7-zip is the best file archiver, and the one with the highest compression ratio that works flawlessly and allows you to open almost any compressed file.

Again, this is open-source software, (under GNU License) and as we've just said, it uses a great compression ratio within LZMA compression, being more stable and more efficient than other compressors. Even though packing&unpacking is only available for 7z, ZIP, GZIP, BZIP2 and TAR; you can use 7-zip for almost all of your uses. However, i've not been able to use it with Al-zip packages, at least yet.

One of the features that I like most is the speed that it uncompresses your files, far better than Winrar or similars. Also, that built-in AES-256 encryption for *7z and zip formats it's one of the most used.

Being open-source, you can check the SDK and of course you can download the source code, and you can even check for support on their forums.

Download: Here.
Beta Versions: Here.
Download version for Windows Mobile: Here.

To be honest, this is my best compressor since I first saw it. Even though i've been using WinRAR, WinZip, and even KGB, this has proven to be the fastest and with better compression that i've seen. Give it a try, it won't disappoint you ;-)