Every Pesach seder I've been to, including ones run by Chabad and Hillel, has used horseradish root as the maror. But this seems to be controversial -- maybe horseradish isn't ok at all, or maybe horseradish leaves are but not the root.
This comment cites the Mishna B'rura (473:42) as allowing. Other comments there say:
If you mean ample late authorities, then yes that is the case. You will be hard pressed to find rishonim who say so, and any who do are likely referring to the horseradish leaf, not root, as the Mishna seems to explicitly exclude the root. See for instance, Haghot Maymoniot 8:13 who notes horseradish as the translation and then immediately says that roots are no good. – Double AA♦ Mar 18 '13 at 5:30
@DoubleAA Yes, I was referring to acharonim mostly. | The Hagahos Maimoniyos (7:13) mentions horseradish ("meerrettich"), as you pointed out. He then goes on to cite Rabbeinu Tam, who I initially interpreted as inferring that the species of maror mentioned in the mishna can only be eaten moist if their roots are eaten, whereas their stalks can be eaten moist or dried out. At least that's what I think Rabbeinu Tam might have been saying based on the context of the gemara, though I didn't see the direct quote. The Shulchan Aruch, though, says explicitly that roots are no good.... – Fred Mar 18 '13 at 6:58
I also read somewhere that horseradish is dubious but has such a long tradition for Ashkenazim to be valid for that community.
So, what's the deal with horseradish? Who permits it, and are there any conditions? Who forbids it outright? It looks like I could assemble a partial answer from these comments, but I'd rather have a complete answer in one place, hence this question.