If you're about 100 kilometers out or more, Twitter! Webcomic xkcd points out how posts about earthquakes can start hitting the Internet within 20 to 30 seconds of the quake, and despite that lag, it's possible for people to be reading about a strong quake before it hits their area.
Check out the comic strip below:
The USGS also has an automatic earthquake notification system that you can set up to tell you about any earthquakes you may be interested in, though there's some delay between the quake and the notification. I found a series of these in my e-mail caused by yesterday's initial quake and the subsequent aftershocks, and just received information on another aftershock from last night in my e-mail as I was writing this.