Although I've said before that Caché's main plot is NOT about who the perpetrator/s of the videotapes is/are, I still like to give my two cents on who might be responsible for the tapes.
(WARNING: Possible spoilers ahead.)
I think these people are the...
(Seriously, you shouldn't read ahead if you still haven't seen the film.)
...these people are the ones that could be responsible for...
(You're still reading. Watch the film first.)
Here are my suspects:
1. Pierrot and Majid's son - Pierrot is sulky towards his mother because he suspects her of having an affair with a family friend. As for Majid's son, he probably wants to avenge his father's misery, which is caused by Georges' selfishness. There you have it. Pierrot and Majid's son both have reasons to pull a trick on the Laurent couple.
The last scene of the film suggests that these two youngsters are conspiring to teach Anne and Georges "a lesson."
2. Michael Haneke - the film director himself is the one who made/sent the videotapes. Haneke uses the videotapes as a plot device.
3. Us, the audience - some 10 minutes into the film, I realized that we — the audience — might be the "sender" of the videotapes. Yes, we are the stalker of the Laurents. Why? Georges never saw any camera when he passed by the area where the camera might be placed.
That's what we do as an audience, we stalk the characters in a film. There's a stalker in all of us.