Main Site: Around 2000 IIRC, before it crashed, anyway. This was back in the days of Digg, and less robust hosting. Average is 150-250 at a time, less at night, more during the day.

Micro-Site: We set up an experimental just-for-fun site with a bit of a viral edge and got 1.1 million visitors in one day, mostly from China - it apparently front-paged on a few major sites over there.