If you have a PowerPC based Mac, like a G5 or a G4, you can borrow the Kernel and Classic related software from a 10.4 and earlier installation. The kernel is the limiting factor in Classic compatibility in Leopard, 10.5.
As stated above, you can make use of SheepShaver which will run most, but not all Classic software. Make sure you use the latest builds from the www.emaculation.com/ web site. With SheepShaver you can install up to 9.0.4. Another option: you can use Basilisk II which will install up to Mac OS 8.1.
For either SheepShaver or Basilisk II, you must have a boot-ROM from specific Macintosh computers. Again check the forums on emaculation.com