Find Someone Who...
I learned this game at the 2005 OCA Conference, at the "No Equipment, No Planning Time, No Problem" session facilitated by Matt Barr.
The group mingles together within the playing space. The leader calls out an instruction such as "find someone who travelled the same distance as you to get here today." The players then have to find someone who matches the given description. This is a great way to get people talking to each other at the start of an event, and works best with a large number of people (20 or more).
Other ideas: