Has anyone used a Linux FS that supports transparent compression? We'd like to mount a partition where all files are compressed and access them through standard POSIX APIs. I know there is an add-on kernel module that extends ext3. Are there other/better options?
ZFS on Fuse. Supports compression on the fly.
Drawback - runs on userland.
Also, you can patch kernel to include zfs support.
As Kristaps says, ZFS does support it.
Your other option is Reiser4, but honestly I wouldn't use that nowadays. It never made it into the mainline kernel. I'm not sure what distros support it or whether it has a future.
Unfortunately, according Wikipedia's comparison of file systems, those are pretty much the only options as far as normal file systems go.
If you go the FUSE way as suggested by Kristaps, you could also use FuseCompress. You won't need ZFS support to use it (you might actually use it with any filesystem afaik), but it will still be running in userland.