There are a number of sites that do variations on what you're describing.

The best of them, though it has issues and is far from perfect, is SpeakerMatch.com. Please understand that I am not endorsing the site, and I think it's not very advantageous to speakers, but it does match speakers with opportunities.

I'm not sure your motive in asking the question, but assuming that you are a speaker looking for opportunities, then the key is not in looking for sites like this, but rather to learn how to effectively market what you do so that when a meeting planner is looking for a speaker, your name comes to mind.

I have to disagree with the last poster on how speakers are selected. When a meeting planner is looking for a speaker, they usually know what topic they want, and often even have specific speakers in mind. That is why MOST speaking opportunities are never advertized, and why a site like you describe can compliment a speakers' marketing plan, but can never replace it.