There are a couple of solutions. In Modeler you can pess F3 to rest an object on the ground on any or all axis. F2 centers the object at the origin.
And there is some Layyout function that does this but I can't think of the name right now. But you can check out these plugins:
http://www.flay.com/GetDetail.cfm?ID=1556
http://www.flay.com/GetDetail.cfm?ID=1414
http://www.flay.com/GetDetail.cfm?ID=1934
Cheers, Florian