Memcached is an open source plugin (available for use under LGPL with the FarCry Commercial Licence and GPL distributions)
Memcached replaces the default object and webskin caching mechanism in Core with an external memcached server. This is an excellent approach for moving memory load out of ColdFusion and off the application server. A key benefit for sites running in high availability environments is that all the application servers in the cluster can be made to share a central cache.
This plugin also works with Amazon ElastiCache.
A large part of the plugin is the reporting - without it, caching is a black box and debugging cache issues become almost impossible. Keep in mind that these reports are performance intensive - applications with large caches (> 100 000 items) may timeout these pages.
Overview - server overview / ElastiCache cluster
As an example where these reports have been useful, you can see in the following screenshot that webskins are being cached properly (their expiries are in the future), while objects all have expiries in the past. This was a bug in the plugin that these reports exposed.