Almost certainly the heater core. Probably worthwhile checking the heater hoses and clamps under the hood - sometimes you get lucky.

This is bad news, as ethelyne glycol is quite toxic. I've read that ingesting it as a vapor is an especially effective way to poison yourself. If it were my vehicle, I would cease using the heater until it is repaired.

Here's the wikipedia reference: http://en.wikipedia.org/wiki/Ethylene_glycol