A couple of years ago I went to see Eben Upton (the founder of the Raspberry Pi foundation) talk about the project. At that time, he had just passed the 2M mark in terms of cumulative units shipped and I remember joking during the Q&A that the killer app for the little computer was likely "gathering dust in a desk drawer."
The thing I didn't realize back then (as is often the case with really useful "glue" technologies like low cost computer that can connect anything to anything) was how important the RPI and all of its clones would become to entrepreneurs and hobbyists alike trying to wire up the physical world. I've seen it go into 3d printers and Internet connected thermometers. I've used it to build a weather-aware Christmas tree in what I found to be one of the most engaging projects I've done with my kids. And inspiring this post, this morning while wrestling with a solution to remote print to my Makerbot, I found OctoPi, a customized Raspberry Pi disk image that turns a Pi into a 3d printer server which allows for job pooling, cloud slicing, and even a video feed.
In short, the RPI feels to me like the mortar of the Internet of Things, connecting all sorts of things to the network in a cost effective and rapid prototyping way that lets people find interesting use cases before having to sit down to lay a board down with a micro controller and a bunch of radios.
[And on that last point: there are a number of other alternatives that can be used to prototype Internet connecting things: the BeagleBone Black, the Intel Edison, the Arduino (which has a very rich ecosystem), and even the new belle of the ball, the ESP8266 (basically a very low cost ARM core bolted to a WIFI chip, <$10 in lots of 1). I've played with all of these and they each have their merits but what they lack is the same rich ecosystem that has sprung up around the Pi (except Arduino which unfortunately is not as general purpose or as connected as the Pi)]
The dark (but awesome) show Mr. Robot even features a Pi as a main character in its first season— I can't quite sing praises at that level but this here is my little ode that cheap little underpowered bit of mortar.