I got a little 128x64 I2C OLED display for $2.50. It said in the product listing that it used an SSD1306 driver IC, which made me think it would work with #QMK firmware.
I hooked it up but it only showed static.
So I read some datasheets and learned some stuff and did some hacking and now (or very soon) QMK will support this OLED display too! (Turns out it actually uses an SH1106.)
Pictured: the wireless receiver for my Mitosis #mechanicalkeyboards, with a speaker and this OLED display attached.