Flickr PHP Messaging Server
This has been linked all over the frickin’ place today, but it’s worth linking to again: Flickr Engineers Do It Offline. Messaging servers and offline transaction or analytical processing is a great way to make your computer bucks stretch. Any time you need to do a process that would require a weighty join between three different sets of information that could span several databases in a sharded environment, it may just make sense to do it offline in a separate cloud and then have the end results fed back into your online environment.
Another couple of places where I know this is used is in games like SpaceHamsters. Both games use a separate cloud (In some cases tied to their online databases, and in some cases not) to handle pre-scheduled state transitions between objects that involve some serious number crunching.
