There's an interesting article on Hamlet skulduggery over at the Guardian - it turns out that David Tennant's been using a real one for the graveyard scene. I rather like the idea of leaving your body to a theatre company, rather than to science...but maybe that's the exhibitionist in me talking.
I once had one of the RSC's prop skulls in a box under my bed, which was a little macabre. It had been Ken Branagh's rehearsal skull for the RSC's 1992 production of Hamlet...so presumably looked a lot like this one:
(via the branaghcompenium.com)
I was stage managing a student production of Hamlet at the time (we're talking 1993, I think), probably due to the fact that I'd actually seen Ken in all four and a half hours of the RSC thing, and had been totally bowled over by it.
Rather pleasingly, the guy who played Hamlet, Adrian Fear, has gone on to become a pretty successful actor (even if he now has far less hair than he used to). He had stage presence, even then.