Just for the sake of thoroughness, I thought I'd mention that you can open Poser's compressed files with most file-archiving utilties, including WinZip.  They use standard zlib compression algorithms.

Dimension3D also has a free utility (Poser Compressor) that will uncompress files.  You can get it here:

http://www.ralf-sesseler.de/d3d/
http://www.renderosity.com/mod/freestuff/index.php?user_id=288865