There are a few Q & A sites that allow people to post questions anonymously. I see on a site called Answerbag many people will post a question and use anonymous as their name, while others will use their name when asking a question. However, on this site, you'll still need to register.
All Q & A sites ask you to register with their site if you want to post a question or answer one. You can always view the site and look through the topic as a site visitor. But if you want to ask anything, you must register.
To keep your identity secret on sites like this most people will create a second email account under a different name. They will use this account to register on these types of sites to ask questions. They can always post their question as anonymous as long as they have created an account on the site first.
All the Q & A sites will require a person to join the site to either ask a question or answer one.