See the test results for all programmers.
Based on visitor data on this site, the STK-200 is the most popular parallel programmer. It uses a 74LS125 or equivalent to buffer the signals. The stated purpose in most references is to protect the parallel port from damage. I can't grasp how connecting one 5V signal instead of another is safer, but that's the claim. One thing it actually does is buffer the signals. The long lines in the parallel cable can cause ringing that puts a bad signal on the AVR SCK line. Putting a buffer close to the AVR end of the cable helps, as does a series resistor. If the ringing is too bad, a very small capacitor - on the order of 11pF to 18pF to ground might also help. This is true of the SCK line on any programmer with a fairly long cable to the AVR.