Recs.
Updated
Not a tool per se, but the file system supports compression of entire volumes with lzo or zlib. There's been proposed others, and some is already in the works. It is also a part of the Linux kernel.
SpecsUpdate
Pros
Pro Good compression
While LZO is by far not the most effective compression algorithm, running it on a per-volume basis allows larger volumes to be shrinked quite well, which in some cases may also increase I/O throughput.
Pro RAID / Drive pooling / Multiple device support
Storage devices of different sizes can be combined transparently into a single storage pool, drives can be swapped out with larger drives, or more drives added to the pool to increase capacity, etc. Support for software-based RAID 0, RAID 1, RAID 10. RAID is supported even with drives of different sizes, on a per-file basis.
Pro Self-healing / bit rot protection
Checksums for data and metadata, automatic detection of silent data corruptions. Can set files to be stored redundantly as multiple copies on the same drive, or spread across multiple drives to protect against corruption. Background scrub process for finding and repairing errors of files with redundant copies.