A friend of mine used to courier a lot. She was from London and living in NYC, and there were a lot of options on that route. Usually it cost her a few bucks for the trips but I recall her flying for free on one occasion.
If you're flexible, live near a big international gateway (e.g., NYC, LAX, MIA, YYZ), and are looking for trips to the "big" overseas spots (e.g., LON, PAR, FRA) you could get some great deals. Otherwise I would guess it's hit or miss.