I'm into AVRs, and already have a collection of pro minis, a nano, full-size arduino clone, and a number of ATtinies - t13a, t85, t84a, & t88. I also have a few different USBasps. I've also got a couple HD44780 displays, some nrf24l01 modules, and 433Mhz ask/ook modules.
I also have a couple esp8266 modules, and have an esp-12e on order.
Any suggestions for something cool and cheap (under $5) to add to a nerd's collection?