If you've never seen this film then I highly recommend it. It's French (with English subtitles) but don't let that put you off.

Synopsis: "At a secluded lake, Alexandre Beck is beaten into a coma, while Margot, the childhood sweetheart whom he has recently married, is abducted and brutally murdered. Police suspicions first fall on Alex himself, but then a local serial killer is apprehended whose peculiar signatures have been found all over the crime scene. Case closed.

 
Eight years later, a still grieving Alex receives one anonymous e-mail with apparently live footage of Margot standing at a crowded train station, and another message warning him: "Tell no one. They're watching."".

It's been voted by quite a few people as the best film of 2007 and I'd have to agree.