Apple posted a detailed list of Mac OS X 10.6-compatible printers and scanners on its Support Web site. The list includes a large number of devices from HP, Epson, Canon, Xerox, Apple and more.
The Knowledge Base article lists specific device software version numbers, whether drivers are provided from the manufacturer or are part of the Gutenprint project, and device print, scan and fax compatibility.
Considering the length of the list, there's a good chance most Snow Leopard users will find that an appropriate driver is already available, but for instances where software isn't available, be sure to check with the device manufacturer to see when new drivers will be released.