There are several other posted here, but I feel that all of them do something which will fundamentally disrupt the flavor (e.g. crockpot will make them soggy and not quite the deep mahogany brown that you want, adding water will do the same). Being a lover of French Onion soup, here's a method that's worked well for me over the past four years or so of experimenting:

1) Choose a pan that has high heat retention. A cast iron skillet would work well for this purpose, although you may run into some concerns when it comes time to deglaze. Generally one deglazes with something like wine, but people recommend keeping highly acidic stuff away from cast iron out of fear of damaging its seasoning.

2) Throw the onions into the pot/pan. Add a pinch of salt and a pinch of sugar to the onions. This will help them caramelize more quickly.

3) Turn the heat up to high and cover them for about two minutes. When you open the lid, you should notice a large amount of steam escaping. This is called "sweating" the onions. Onions need to be dry in order to brown properly--the sweating method helps them dry out more quickly.

4) Turning the heat down to medium-high, stir occasionally. You'll stir more frequently towards the end, pushing up from once every two minutes to once a minute, to once every thirty seconds, to constantly near the end. They're done when they're a deep mahogany brown.

That's it, really. As the original author said, there aren't any shortcuts to proper caramelized onions. The salt+sugar will help ever so slightly, the sweating will help ever so slightly. But you really shouldn't expect to caramelize them in less than a half hour.

EDIT: Down-votes for posting a recipe on HN? That's gotta be a new one on me.