I talked at some length about it in my recent post on Buddhist use of entheogens. I'd be happy to expand further tomorrow afternoon if need be. The short answer is that there is no specific academic consensus, and there may never be since it would be difficult to identify it without more concrete texts. Amanita, psilocybe, ergot, and marajuana have all been proposed, but I think the most likely candidate is ephedra, which is a stimulant rather than a psychedelic and thus closer to the Vedic description and still used as Haoma today by Zoroastrians.