NOTE. OPTIMUMCACHE IS NO LONGER SUPPORTED.
OptimumCache is a de-duplicating file cache optimized specifically for shared hosting. Typical shared hosting server runs a number of sites with WordPress and Joomla as well as other popular software. This usually means that there are hundreds of duplicate files that are constantly being read into file cache - both wasting precious disk IO operations as well as memory. OptimumCache creates a cache of such duplicated files and de-duplicates file cache.
With OptimumCache, if a duplicate of an already loaded file is requested, the file gets loaded from filesystem cache. By doing that, system bypasses disk IO, significantly improving the speed of reading that file, while lowering load on the hard disk. As the file had been read from disk just once, it is cached by filesystem cache just once, minimizing amount of duplicates in file system cache and improving overall cache efficiency. This in turn reduces memory usage, decreases the number of disk operations - all while improving the websites response time.
Further reading: http://kb.cloudlinux.com/tag/optimumcache/