Seamless file compression has found its way to Mac OSX 10.6 with Squeeze.
The application is a system applet, implementing the new HFS compression technology to transparently compress folders you configure to be compressed. Each folder added with Squeeze will be compressed in the background. To the user, they are just regular files, but they will take up less disk space.

Using Squeeze is easy: just select the folders you want to get compressed, and it will silently work in the background to compress the contents of those folders. Whenever you use any of the files, it will be read like any other file, just like that. Compression occurs completely transparently.
Additionally, Squeeze keeps track of space gained, allowing you to trade off performance vs. compression effectiveness. And, as Squeeze is in fact a front-end for compression technology built into Snow Leopard, compressed files will continue to work even after you uninstall it.
More features can be found on their homepage: http://www.latenitesoft.com/squeeze/.

They also have a demo version available, allowing to test-drive Squeeze (gain of up to 50 MB).