Great free archive utilities are pretty hard to come by on the Mac. Keka is the exception. If OS X's built-in compression utility doesn't do enough for you, Keka could be the answer.
Keka's been around since 2009, but only recently made it into the Mac App Store. Unlike Growl, Keka is available both as a paid, US$2 app from the Mac App Store and as a free download from the Keka project site. The two versions will be kept in sync, so there'll always be a free option. Of course if you use it, supporting the developers, either by a donation or buying it in the Mac App Store, will help them continue to update it.
Keka allows you to compress files in multiple formats. Zip, 7z, Tar, Gzip and Bzip2 are all included. What's more Keka can extract even more with RAR, 7z, Lzma, Zip, Tar, Gzip, Bzip2, ISO, EXE, CAB, PAX, and even ACE supported.
Keka also supports encrypted archives meaning you can password protect compressed files, as well as split archives -- handy when you have file size limitations.
Like the Unarchiver and others, you can set Keka as the default extraction client, so files can be decompressed with a double click. If you want to get a bit more advanced, the Keka Dock icon serves as the one-stop shop for compression. Drag and drop files onto it to either decompress or create a new archive in the default format of your choosing. Format and compression method can be changed quickly from a pop-up menu from the Dock icon. It's all very slick.