For most compression and decompression needs, StuffitExpander is all I ever use but there are occasions when it can't handle certain file types.
GUI Tar Extractor decompresses and extracts files from a huge number of archives including .7z, .tar, .tgz, .tar.gz, .dmg.gz, and many more. In addition, what makes it a little different to other similar apps is that it can archive files and folders from multiple locations instead of being restricted to just one directory. This is very useful if you've got compressed files in several locations or what to decompress them to multiple locations.
GUI Tar Extractor is very easy to use. In extractor mode, simply browse to the file you want to extract and the directory you'd like to extract it to. In compressor mode, use the plus symbol to add a file and then simply click Compress. GUI Tar Extractor is very fast, although there's no progress bar as such to let you know how far the process has advanced.
GUI Tar Extractor is a very simple but powerful tool for almost any compression or extraction task.