I have infinite health (due to hacking with WorldEdit, or possibly TooManyItems, I'm not sure how I did it). This is on an adventure map, so this is not desirable. How can I reverse the effect?
After reverting any mods that may be resetting your health, the easiest way would be to drown or sit in lava. Most mods that give you "infinite" health simply set your health to some ridiculously high value so that it is extremely unlikely you will ever run out. Because drowning/sitting in lava constantly damages you, it will require far less effort than repeatedly falling.
Alternatively, if you still have WorldEdit loaded, you could build a tower up to layer 128 and carve a hole to bedrock and fall the greatly exaggerated distance. It will probably take several falls to knock off all the extra health you were granted.
Note that either method can be sped up considerably by installing a mod, such as Single Player Commands, which will allow you to set your health back to a much more reasonable number.