I want to highlight libarchive, since I have used it in a recent project and I was very impressed. The developers were immensely responsive and help to fix any issues that came along. The architecture is a very elegant stream orient API that almost begs to be wrapped in higher level languages. I've in fact wrapped libarchive in Scheme (ChezWEB) so that it's available via the Arctic Repository (I'm working on a new release). It's not complete, but I've absolutely loved working with the libarchive code, which, horrors of horrors, is actually readable! What more could you ask for except for a well documented, well coded, and active library that actually handles almost all of the archive formats you could want automagically?
Kudos to libarchive.