Quoted from the wisegeek article above:
"Unacceptable methods of euthanasia are: freezing, boiling, chopping, removing the fish from water, using a seltzer tablet, slamming, pithing, decapitating, or flushing down the toilet. These methods are slow, torturous, stressful or violent. Clove oil followed by vodka is both inexpensive and humane. The fish goes to sleep like we might before an operation, and simply doesn't wake up."
Freezing is slow.
As a fisherman, I find a quick whack on the head works well - stops the nervous system very quickly! Although not for the squeamish or young. If you land a 20lb fish, you're not going have the room or time to use clove oil (although probably taken out of context) on a bank or boat.
Clove oil does look the best solution in the domestic/aquarist scenerio.