This is being treated as a "naked lie" of Palin on this site, but it seems that she in fact did stop this bridge, at least according to CQ Politics.
However, it remains untrue that she "told Congress 'Thanks, but no thanks'". A summary is below...
UPDATE: Talking Points Memo points out that she only stopped the bridge project after it became clear that Alaska itself would have to pay $329 million for it. That puts rather a different light on things. Thanks to Trix for pointing this out!
In October 2006, while campaigning for governor, she supported the bridge. At this time, Congress had already decided not to tag this money specifically for the bridge, so she did not have to do this. Instead, Alaska could build whatever it wanted with the money, including the bridge to nowhere.
Only after she became governor in 2007 did she decide to use the money in a different way and not for the bridge. The article does note that at this time, it would have been "an uphill battle" for Palin to get the bridge built. But she could still have tried to get the bridge built, and she decided not to.
So it is really not a lie that she stopped the bridge. It was her decision not to push the matter further at this point. But she also did not give the money back to Congress. (Indeed, who in her place would?) And she certainly did not tell Congress to forget about the bridge! So there is a lie here, it is just not (exactly) the lie that is being reported in most places.
Anyway, I thought this would interest you.