Boiling coolant

Someone else noticed coolant boiling out of our car this evening.

Towed it home and looked up details online; it seems like one of the following:

1. Water pump (replaced July 2010...unlikely)
2. Bad hoses...seem to check out OK
3. Bad thermostat (quite possible)
4. Bad radiator cap
5. Fan relay
6. Blown head gasket

Hoping not the blown head gasket.

UPDATE: mechanic replaced a burned-out fan motor and partially melted relays.