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microtherion has published new project the ChipHeadBang, that is available on Github:
Design for an USB to Serial converter with ICSP header for (slow) bitbang programming, based on the CH340G chip.
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
