Compress is a freeware application from a good ways back. I originally wrote it just to have a GUI front-end to p7zip. Once I wrote the wrapper for p7zip, I found it was fairly easy to add support for tar, gzip, zip, and bzip2. Once I had those in place, I decided to make it available to see in case other people found it useful. Well, it turned out that a lot of people liked it so I've been steadily improving it since then.
I've never bothered to put a license to Compress's source code. I consider it free for the taking. Although I would appreciate a mention in an about box if you ever use anything. A lot of the code is legacy from when I first started programming so a fair portion is pretty bad but it at least works.
Compress makes use of p7zip as compiled by MacPorts. p7zip is under the GPL licence. Since the GPL is viral, it may or may not have anything to do with my source. Since I don't use the source but instead simply call the application, I think I've skirted the GPL. That said, I'd appreciate if someone more informed could let me know one way or another.