The G is for Gearman - Gearman is a system to farm out work to other machines, dispatching function calls to machines that are better suited to do work, to do work in parallel, to load balance lots of function calls, or to call functions between languages.
The extra M is for Memcached - memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.
More and more these days, you can't run a web site on just LAMP. You need these extra tools (or ones like them) to do all the cool things you want to do. What other tools do we need to work into the acronym? PostgreSQL replaces MySQL in lots of stacks to form LAPP. I guess Drizzle may replace MySQL in some stacks soon. For us, it will likely be added to the stack. Will that make it GLAMMPD? We need more vowels! If you are starting the next must use tool for running web sites on open source software, please use a vowel for the first letter.