Background file compressor (formerly known as Squeeze).
Clusters is a background file compressor, which uses FS-compression technology to transparently compress the folders you configure it to process. OS X will read those files normally, they will just take less disk space.
What's new in Clusters
Fixed a couple of issues with the menu icon when running on OS X 10.9.
Important: Running Clusters version 1.6.x (or earlier) on OS X 10.9 may trigger a serious incompatibility problem that could cause some of your files to appear as if they had zero bytes (your data is still there, though; it just looks like it cannot be accessed). If you used an old version of Clusters in any of the Mavericks developer betas and run into this issue, please upgrade and contact email@example.com so we can help you fix the affected files.
The menu icon can be hidden again.
When using multiple displays, the menu icon no longer appears several times.