SparkFun AVR Programmer HAT
The SparkFun AVR Programmer HAT lets you program AVR microcontrollers (the chips you'll find on Arduino boards, for example) directly from the SPI hardware pins on the Raspberry Pi.
It was originally an in-house solution for production programming, but has been tidied up into a programming tool for anyone.
It's fully open source and hackable, works directly with AVRDude and provides ISP programming which is great for deployment since it doesn't require a bootloader, uploads code faster and allows you to set protection fuse bits. It's useful if you're programming many AVR chips either for hobby projects or small yield production.