If you haven't seen the movie this _might_ spoil it. It's the alternate (and I think original) ending ...
If you have seen the movie, watch it; it's more inline with the original story's ending.
I first saw it here:
http://www.slashfilm.com/2008/03/05/i-am-legend-alternative-ending/
... and it appears that it's now down at /film ...
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