PeaZip is a free open source archiving program for BSD, Linux, and Windows which supports many different compression and package formats. It also has customizable Windows context-menu integration. It is comparable in power to 7-Zip and vastly superior to older compression programs like WinZip and WinRar. It has a bit more format support than 7-Zip, although 7-Zip can open several formats that PeaZip cannot. It is written in Free Pascal.
A co-worker of mine mentioned how much he liked PeaZip, but, after seeing him show it off, I didn't find it to be any more useful than 7-Zip, and I found the interface to be pretty awful, so I never bothered with it. However, after seeing a bug in 7-Zip when unpacking the latest version of tarballs, I found that PeaZip was able to unpack them correctly. I almost never use the program, except to get around that bug.