I am looking at AC adapters for my Canon 50d. The Canon original is several times as expensive as some of the aftermarket options that I have found. Do I run any significant risk of damaging my camera if I use an Aftermarket Ac Adapter?
As long as the proper voltage and current are provided, there shouldn't be any issues. The thing that may be missing however is quality. There are a number of different ways to adjust voltage and current and the cheapest adapters generally use cheaper methods of converting the power that are more prone to error or failure.
More expensive adapters tend to handle load more consistently and provide cleaner power, but it may very well not be necessary for your needs. Additionally, there is still normally a decent mark up on the OEM adapters, so it's entirely possible that a third party adapter of equal quality could even be half the price, but it's always a dodgy game of figuring out which ones are good and which ones are not.
Yes, and no.
Yes, you can damage the camera if you are using a low quality power adapter that for example delivers the wrong voltage.
No, you don't damage the camera just because a power adapter is not made by the same company as the camera.
One thing that you should check however, is how it affects the warranty of the camera. If using a third party power adapter voids the warranty, it's not really worth it.
You run a higher risk of harm to your equipment, your home or yourself. You may be lucky and get a decent quality power supply but the odds are against you if you buy a cheap unbranded supply.