1.08 volts on my Northwood Celeron 2.0 GHz. Stock is 1.5 volts.
When I had a Sempron64 2800+, it did 1.1 volts at stock speed.
2.0 volts on this K6-2. Stock is 2.4 volts....unfortunately it can only run up to 350 MHz at this voltage. Not that the extra 100 MHz would make the computer any faster, as it is currently not the bottleneck.
I never tried to see how low my Celeron D 310 could do when I owned it, but it would clock to 3.6 GHz on stock voltage, and one time when I messed up a socket after pin modding, it was able to still post at 3.2 GHz on just 1.1 volts, stock being 1.35 volts (crashed when loading OS of course).