Lately, I've been playing a lot with MongoDB's group command. It's a powerful command that allows you to write some really interesting queries quickly. Here's an example of one that calculates "Tags" on a blog site:
In my tests it also proved to be about 4x faster than a similar MapReduce, however, it comes with a severe cost: It blocks all reading from the collection. This is a huge problem and basically makes it worthless for doing serious queries on a database with say hundreds of thousands of users like I have in my day job. From what I can tell finds, distincts, and mapReduces don't block and some combination of those provide non-blocking alternatives.
Here's a simple map reduce example:
One more note, I recently read MongoDB: The Definitive Guide, which had a lot of examples and clarification that are not readily available in the online documentation. I highly suggest checking it out!