Objective Development's USBaspLoader (orig. http://www.obdev.at/products/vusb/usbasploader.html) with my patches.
C Assembly
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usbdrv
.gitignore
Makefile
README.txt
loaderconfig-default.h
loaderconfig-usbasp.h
main.c
usbconfig.h

README.txt

Rename loaderconfig-DEVICE.h to loaderconfig.h for everything to work.
-usbasp is configured for USBasp programmer pins,
-default is for OBDev.at defaults (D-/D+ on D0/D2, no pullup).

Copy the -default header and modify it if you need a custom setup.