If you go into real computers, the C.H.I.P.[1] is quite an interesting device. It's $9, which is price competitive with the ATMega644 chip (in single quantities), has WiFi, Bluetooth, and runs Linux. It's also got lots of GPIO, like the Raspberry Pi, so it can be used in places where a microcontroller might normally be used.
I'm sure there are others, but these are the two that are on my radar at the moment.
The peripherals are so much better it's comical. Basically all pins capable of 16-bit PWM, having dual 12-bit ADCs, a real DAC, and removing most of the extra junk? Amazing.
Now I just use it as a plug in brain for my custom boards, since it takes all the hardest to solder parts for an entire SoC and puts them in a form factor that's quite small and appropriate power for anything I've wanted to do so far.
I do want to do a project using a teensy with the audio board for beat detection eventually, but the teensy audio library still doesn't suppert beat detection. Paul posted that he would be adding it a couple of years ago I think, but it currently isn't implemented.
ESP8266 for ~$3 at 160MHz with wifi built-in is such a bargain I can't go past it most of the time, and when I don't need wifi and want to use the Arduino libraries, Arduino pro micros are about the same price and have USB HID support.
It's so great that there are so many great and well-documented options out there nowadays!
[1] http://www.instructables.com/id/Ultra-bright-LED-Color-Chang... (things have come a long way since 2010!)
Program in JavaScript, from a web IDE even. Wireless uploads to the board. $19.
They are a really interesting IoT play becoming and MVNO and all!
Also -- they had one of the funniest kickstarter video's i've seen: https://youtu.be/-sP5hn6VDgE
> The ESP8266 is a low-cost Wi-Fi chip with full TCP/IP stack and microcontroller capability...
Still, it's pretty cool you can buy one for like $2 and basically have a computer that runs Python with some minimal microcontroller functionality.
I've only used the Arduino libraries, but I haven't had a problem yet.
Funny that a 160MHz board with wifi built-in can be referred to as "minimal microcontroller functionality" nowadays.
This allows you to port real operating systems to it:
http://cowlark.com/2015-10-27-fuzix
It's also a really elegant architecture which is a pleasure to write assembly for, and it consumes absurdly small amounts of power.