In Maya 8.5 i would have to say nCloth and the new nucleus system.
Gster and I are currently using this in a wee project (sorry don't ask as I wont tell you what it is, really secret 
). It has become indispensible and is a superb new addin in to the new Nucleus system Maya is having build for its new dynamics system.
Cant wait to see the release of 9 :attn:
Chris (formerly R@nSiD)
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Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)