Clove oil is my choice - you can usually find it at the drugstore for toothaches.
From Jay Hemdal's post in the fish euthanasia sticky in the disease forum:
"
Clove oil (eugenol)
One product that aquarists can buy that is approved by at least some veterinarians and research biologists is clove oil, also called eugenol. A dose of 50 mg/l is usually sufficient in euthanizing fish. This equates to about 0.20 ml of eugenol in one gallon of aquarium water.
Because eugenol does not mix well with water, and because larger volumes are easier for hobbyists to measure out, it can be dosed using the following method:
1) Add 2 ml of eugenol to 100 ml of tank water in a sealed container and shake it vigorously.
2) Add 10 ml of this suspended solution to each gallon of water needed to euthanize the fish.
3) Place the fish in this solution and keep it covered to prevent it from jumping out and to help keep it calm.
4) Wait at least twenty minutes after it stops breathing, and then remove the fish from the solution and freeze it for later disposal.
"
