I'm sure someone on here knows of some webapp that lists these things and when they're taking place.
I appreciate the help!
The real answer is that those aren't terribly likely to have specific events as those events generally arise from existing communities. Simply put, you should be using Meetup and Lanyrd to get involved in your local community, and by being active in those communities you'll almost certainly be in the know for future hackathons.
Where I'm at (Annapolis), I'm kind of in a middle ground between two big metropoli, DC and Baltimore, so I maintain membership in local groups and Facebook groups that I generally don't show up to much (sorry everybody) because they're slightly further away from where I want to be, but by being involved in those groups, I hear about pretty much everything.
Another option is of course to ask here, and specify your location, and you'll likely here about something.
Edit: I don't know where you're at, but a great way to get plugged in (if your area has one) is to attend a StartupWeekend or two, even if you have to travel a bit to attend one. In my experience, they're run by avid technologists that have substantial connections in the tech community. At worst, you'll find out about a Facebook group or mailing list to join, at best, you'll get introductions, face-to-face meetings, etc.