Most interested in peer-to-peer solutions - without central server. So, I imagine it like a library that brings to my application a functionality of transparent cache management with feature of remote instances synchronization. It should support cache record timeout and forcing invalidation.
UPDATE: If not completely peer-to-peer - at least with multiple servers feature.
The next best thing after Memcached is Redis:
(+) it supports more data types;
(+) has persistent storage;
(-/+) has a few C++ clients that seem active (as of 09/2018) but none of them are "recommended".