Ambac and MBIA, of course. The estimate of downgrades comes from Bloomberg.
(Moodys will no doubt follow suit soon.)

And the FT (among others) points out that weren't we all panicking about the apocalyptic consequences of a monoline downgrade back in Q1, and what about that then? Well, yes we were; and since then we've had the ARS shutdown . But there's an argument that the effect of the downgrade has already been priced in - in which case, yesterday's decisions simply emphasise how far behind the curve the rating agencies are.