Absolutely. HFS+ has a built-in compression just like NTFS does. In fact, it's on some apps (anything that installed with Snow Leopard for starters; i.e. iCal, iTunes, etc) by default. Put simply, Squeeze is more of a frontend for this feature. I do believe that you can get the same results with terminal commands, but Squeeze does two other things that the terminal command wouldn't be able to: live updating (if you have a folder on the list and add an item to the folder, Squeeze will automatically compress it) and exclusion lists (it will be default ignore formats that don't compress very well or at all to save you time - such formats include MP3, AAC, AVI, MOV, you get the idea).
To run something that's compressed, you don't even need squeeze. That particular ability is coded into the system by Apple and they seem to trust their own tech enough to bundle their apps with it.
䷟䷽䷏䷁ — Dying of the Light.