EveryMac has published a comprehensive list of those Macs that can boot Mac OS 9, those that can run Classic in Mac OS X and those that can do both. Systems are noted that can run Leopard, however, must stay in Tiger to run Classic. Leopard has no support for Mac OS 9 Classic mode on Intel or PPC.
Users are now faced with multiple scenarios in terms of running Mac OS 9 applications. There are those Macs that can boot into Mac OS 9, there are Macs that can boot into 9 or run Classic, there are those that can only run Mac OS 9 applications in Classic mode, there are those that can only run Classic in Tiger or earlier but could run Leopard, and finally no Macs can run Classic at all if Leopard is installed.