Raspberry Pi Stand-Alone Programmer
QCPete 
Enter the Raspberry Pi - Programming Over GPIO
After exploring the chip to chip programming option, and finding that it would not work for our application, we searched for another solution. This time we wondered if a Raspberry Pi might do the trick. A quick search of the internet lead us to a great tutorial by Adafruit that uses GPIO to "bit-bang" programming to AVR chips.
We got this up and running quite quickly. Stoked! Made a custom hat, so we could run this headless, and started migrating some of our kit programming procedures. A little slower than the MKII, but it was completely stand-alone and very reliable - I'm in!
My latest @Raspberry_Pi project: stand-alone AVR programming @sparkfun. 3 live in production. Only 100 more to go :) pic.twitter.com/6ci5N5gcCI
— QCPete (@qcpete) September 6, 2016
Then I found that a second or two longer is actually a lot of labor and production was really not digging this.
Looking on a scope I saw the clock signal flying at 84K. This was leading to some programming times of around 4 or 5 seconds, and then an additional 4-5 seconds to verify. Unfortunately, this was not gonna fly in production and we needed to find a faster solution.