I’m a self confessed pcb hoarder, @Janet and @xquared are looking for smd projects, I thought they might not be the only one so I’ve done an inventory.
My favourite is the SAMD21 board. You can do micropython on it (see circuitpython), you can put the microsoft uf2 drag and drop bootloader on it, you can use it as an Arduino Zero.
5cmx5cm and less $1.50 each
Adafruit LiPo charger
Sparkfun SAMD21 Mini breakout clone (same schematic, 2 layer pcb)
Generic ATTINY85 carrier with regulator and output FETs
SparkFun nRF52832 Breakout (Nordic Bluetooth SoC)
smdprutser black magic probe mini v1
Adafruit 8-channel Bi-directional Logic Level Converter - TXB0108
SMD Star
USB Isolator (have free chips too)
Two Tube Nixie carrier
Sparkfun FT231X breakout (USB to serial ftdi)Dangerous Prototypes USB IR Toy
Sparkfun attiny programmer
Micropython pyboard
Sparkfun pocket avr programmer
VHS IoT ethernet+arduino
SparkFun 16 Output I/O Expander Breakout - SX1509
SparkFun MiniGen - Pro Mini Signal Generator Shield
PowerBoost 1000 Charger - Rechargeable 5V Lipo USB Boost @ 1A - 1000C
SparkFun ProtoSnap - Pro Mini
Greater than 5cmx5cm $3 each
Adafruit Retina iPad to DisplayPort Driver
Sparkfun EL Sequencer
ICEZero fpga board for Raspberry PI