Design for an USB to Serial converter with ICSP header for (slow) bitbang programming, based on the CH340G chip. See http://samelimmat.blogspot.com/2015/04/introducing-chipheadbang.html
While this seems to work pretty well, and the CH340G can be obtained extremely cheaply from sources such as AliExpress, driver support can be a bit iffy. For current versions of OS X, the vendor provided driver will cause kernel panics, and as far as I know, only this commercial alternative will work: https://www.mac-usb-serial.com
Boards can be ordered at OSHPark: https://oshpark.com/shared_projects/xvqfIt55
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