I agree with using a key in a pinch. A house key is best; use the point(s) near the end of the key and stick them under one of the parts sticking out on the bottle cap and force it up. Repeat a few times until the cap is loose enough to pull off. Actually works pretty well.

OT
I remember in Siena, Italy one time; I had a bottle I needed to open and didn't have a key (the hotel holds it for you during the day), so I improvised with one of the trash cans in Il Campo. The waste bin had a meshed metal casing around it and I simply stuck the cap end of the bottle in and pulled; worked perfectly and it was a refreshing drink sitting there in Il Campo soaking up the sun.