In the episode "Meet Kevin Johnson" Tom told Michale that Widmore put a phoney plane on the bottom of the ocean.

In the episode "The Other Woman" The captin of the freighter tells Sayid and Desmond that Benjamin Linus staged the Ocianic flight 815 crash.
Here is why I think it was Ben who staged this crash.
- Ben dose not want anyone to find the island
- We know the "Others" can go to and from the island
- Tom had the paper work that indicated the fake cash was set up. (Not Widmore)
- He might have not done it phisicaly himself but he was in charge
Here is why I think Widmore did not stage the crash

-He has no motive
And here it is... the big fact that gives proof to this idea. Drum Role please...

- If Widmore Planted 324 so called "Passengers" at the bottom of the ocean why would he even think about rescuing survivors of the real crash.
- He would be put in Jail if they found out he staged a plane crash.
- All they need to find that out is one survivor (and they get six)
If however Ben did indeed stage the crash... then Widmore would be more than happy to rescue survivors and proove Ben was the one who did that
- Think of what Frank said to Michale...Could you imagine if we actually found survivors to Flight 815.
Widmore is a jerk. And he may be up to no good. But he is not stupid.
And he did not stage the crash!