Appropriate to the devastating earthquake that struck off the Chilean cost (and the aftershocks in the area) it makes sense to highlight the tsunami monitoring network that was able to warn folks across the Pacific of the potential heavy waves coming their way.
For the U.S., the place to start is the tsunami section of the National Oceanic and Atmospheric Administration (NOAA) website. NOAA administers two tsunami warning centers, one in Alaska and one in Hawaii. The Hawaii facility also serves as an international tsunami warning center. Detecting the waves falls to tsunami buoys, earthquake detection tools and tide tables. The network of buoys was expanded following the 2004 Indonesian tsunami, with thirty-nine currently deployed in the Pacific. Similar networks monitor the U.S. Caribbean and Atlantic coasts.
There is also a Chilean project for tsunami monitoring. As you can imagine, trying to access those websites right now isn’t possible.