NOTE: I've combined all my various writings about tsunamis from December 2004 and January 2005, including this entry, into a single tsunami article, which might make a good introductory reference.
This excellent animation (much larger 3.6 MB QuickTime version and 760 KB extended-time GIF version also available) from the U.S. National Oceanic and Atmospheric Administration (NOAA) shows why Sri Lanka, in particular, was hit so hard by yesterday's tsunami: the quake was not a focused point on the ocean sloor, but a north-south line near Indonesia, and the most intense wave moved directly west, unobstructed, across the Bay of Bengal to Sri Lanka and southeastern India. That also explains why the wave could reach Africa and still kill people thousands of kilometres and several hours away.
UPDATE: The Wikipedia, which is collectively written and edited by anyone with a web connection who wants to contribute, has turned into a stupendously thorough news and information resource about this disaster. In addition to the links below, it includes a series of other graphics that help explain the event, and the main news article has been edited more than 500 times, with each change refining and deepening the coverage with material from all over traditional and non-traditional media worldwide. It is the best starting point for finding out about the catastrophe by far. Darren Barefoot also has some good links to first-person accounts.