The bombing that killed Hariri was large and complex enough (especially given the degree of security surrounding Hariri) that I doubt it could have been the work of a low-level or independent cell.  And in the context of the series of attacks, there is a clear pattern of targeting critics of Syria.  It would take a lot to convince me that (as some have argued with little evidence) this is March 14 (or their allies) killing off their own people; that theory's really far-fetched, especially on this scale, and I've seen nothing but wild speculation to back it up.

I do think it's possible that not all the attacks are the work of the same people.  We know very little about the forensic evidence collected in any of the killings except for Hariri's, and so it's hard to say from a civilian perspective how many of them are really connected.  But these are sophisticated attacks carried out in an atmosphere of intense security; it's obviously not just some disgruntled teenagers who found bomb-building instructions on the Internet, these are people who know what they're doing.

I do not think it's Hezbollah, full stop.  (Or any of the other domestic opposition groups, for that matter.) Hezbollah's internal legitimacy derives from its claim to be the Lebanese resistance against Israel.  It would have little to gain and much to lose were it to turn its weapons on other Lebanese.

There are Sunni extremists based in more than one of the Palestinian camps, so they remain a possibility, but the logistics of getting large, sophisticated bombs in & out of the camps would be quite difficult; some of them are virtual prisons, and the vehicle entry/exit points are tightly controlled in all of them.  And someone bigger (and outside the camps) would still have to be either supplying them with explosives and other materials, or facilitating their procurement.

In short, I dunno. :-\