That bookmark service that did great out of the del.ici.ous fiasco (sorry I honestly can't remember your name!).
trunk.ly? They certainly weren't the only ones but were in the right place at the right time to attract a heap of users and as a consequence did some crazy weeks to bring forward a heap of planned features to keep the newly attracted users.
Reddit having to work crazy to take up the exodus from Digg.
That bookmark service that did great out of the del.ici.ous fiasco (sorry I honestly can't remember your name!).
Perpetuum having a mass of new players because of the Eve monocle incident.
I'm sure others can think of times when one business has had to react rapidly to either manoeuvres or failures of another business.
Not quite put out of business, but massive opportunity cost if the reaction is not made.