This module creates a cached version of each block. Block caching happens separately from page caching and so it will work for logged-in users whether or not page caching is enabled for the site. Cached blocks make just one call to the database and can therefore reduce the overhead required to render blocks that require complex queries and/or calculations such as some Views or Taxonomy-related blocks.
When this BlockCache is enabled, administrators will see a duplicate "cached" version of each of the site's blocks. By enabling the cached version (and disabling the original version), you can display a block that is physically identical to the original, but is loaded from the cache rather than calculated for each page. This can greatly speed up the rendering of a page. There are options for how and when the cache is refreshed and whether it is cached for the whole site or on a page-by-page basis.
This module is functional, however it is needs a few issues ironed out before an official release for D5 will go out. The D4.7 version is no longer supported and a D6 version will not be created since block caching is now in core.