One of the best tools in ages to show up in the open source world for pure scalability concerns has been memcached. If you haven't learned about it yet, figure it out. If you do know about it and, like me, are in various stages of actually using it and getting your developer colleagues to work with it and understand it, I have a great article for you from Socialtext.com. There, they have an FAQ about memcached which is excellent. So, without further ado and without repeating great data for no good reason. Here is the source. It's well worth your time to visit the site.