Facebook recently made opensource, osquery. It gives you operating system data via SQL queries! Its very neat, and you can test this even on MacOSX (it works on that platform & Linux). It is by far the project with the most advanced functionality, linked here in this post.
I noticed that rather quickly, there was a PostgreSQL project, called pgosquery, based on Foreign Data Wrappers with a similar idea. (apparently it was written in less than 15 minutes; so a much lower learning curve than the regular MySQL storage engine interface)
I immediately thought about an older MySQL project, by Chip Turner (then at Google, now at Facebook), called mysql-filesystem-engine. This idea was kicking around in 2008. I was intrigued by hearing about this at a talk (probably at the MySQL Conference & Expo); it’s a pity no one took this further.
On a similar tangent, did you also know that there is the option to use MySQL as storage via FUSE (see: mysqlfs)? An article by Ben Martin shows some practical examples.
At its heyday, MySQL had many storage engines (maybe around 50). Wikipedia has an incomplete list. I see some engines on that list, and think that some of these folk are also creating MongoDB backends – competition. At MariaDB we are probably shipping the most storage engines of any MySQL-based distribution, however I think we could be doing an even better job at working with upstream vendors, and figuring out how to support & augment business around it.