The most common place is Web MD, but there are other sites as well. You have The Doctor's Lounge and many many more. A quick search will give you a very long list of places you can ask questions and get a response from a wide range of doctors.
It's best to ask a doctor in person or call a doctors office because with these online ask a doctor question websites you never know if you're taking to a real doctor or if you're just talking to some John Doe who is paid to answer your questions.
There are many places you can go to ask a doctor questions. The ones I have found are sites you have to pay for an answer from a doctor. There are also places on the web you can go and find general information for free such as WebMD or .