http://www.apple.com/rosetta/ this software, which is built into OSX, should be able to run any software that was designed for the PPC Macs on the new Intel Macs.
doesn't OSX provide a "Classic" mode for older software? also, are you trying to run older 68k Mac apps? EDIT: Sorry I didn't read your question properly. The only way to run classic apps on an Intel Mac is to use an emulator; http://www.atpm.com/12.09/classic.shtml there is no native support in Mac OS for classic software anymore.