Starting today, you'll be able to send a text message to 911 in an emergency situation. The service will be supported by AT&T, Sprint, Verizon and T-Mobile. Text 911 is only available in a small set of select cities in the US to start, however it will be rolled out everywhere by the end of 2014.
Using the service is easy, just send a new text message to 911 with the details of your emergency — though you'll need to remember to send your location as pinning things down over text isn't the same as it is with phone calls. You'll also be able to send a photo along with your message, though that feature isn't supported across the board just yet.
You can read more on the Text-to-911 service at the FCC's website.