I like the Vista CNC USB spindle tachometer. It's optical as many suggested before, and being USB it doesn't require a BOB input. It is perhaps a little pricey at $65, but was well worth it in my opinion because after I had a bracket to mount the sensor it took all of 1 minute for RPM to be displayed in Mach. Time is money.
Edit: the Vista CNC documentation as I recall claims the sensors will work up to something like 30k RPM, but it is confirmed working up to 6k RPM on my machine.