If you are a web developer then you must know Memcached. It is a very good caching system targeting to speed up dynamic web application. You better use it on web application if you are using LAMP similar solutions.
PHP has already integrated Memcached into its package as an PECL extension to provide API for communicating with memcached servers, it is call Memcached for PHP. It has lots of useful new functionality, some of them I found very interesting are getmulti and cas (compare and swap).