memcached is described as 'Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load'. There are more than 10 alternatives to memcached for a variety of platforms, including Linux, Windows, Mac, Online / Web-based and Self-Hosted solutions. The best alternative is Redis, which is both free and Open Source. Other great apps like memcached are Varnish (Free, Open Source), Cloudify (Free, Open Source), Hazelcast (Free, Open Source) and GridGain In-Memory Data Fabric (Paid).
A distributed in-memory key/value data store with optional schema. It can be used both as an embedded Java library and as a language-independent service accessed remotely over a variety of protocols (HotRod, REST, Memcached and WebSockets).
XCache is a fast, stable PHP opcode cacher that has been tested and is now running on production servers under high load. It is tested on Linux and FreeBSD and supported under Windows, for thread-safe and non-thread-safe versions of PHP.
eAccelerator is a free open-source PHP accelerator & optimizer. It increases the performance of PHP scripts by caching them in their compiled state, so that the overhead of compiling is almost completely eliminated. It also optimizes scripts to speed up their execution.
BigMemory: a commercial in-memory data management suite for real-time Big Data applications that stores up to hundreds of terabytes in distributed memory across multiple servers without performance hits from the Java garbage collector.