Lanyrd, a web app for tracking conference just launched.
It's new, probably still has a limited audience, but it can't hurt to add your event.
I would also suggest contacting meet up groups in your area (web, design, whatever -- any topic freelancers would attend) and asking the organisers to do a small announcement for you, or at least post on the mailing list. The people that attend these events are likely to go to conferences, too.
For a large-ish meetup group you could consider offering a free ticket as a door prize.