If you’ve upgraded to Leopard but still need to test a site in an older versions of Safari, you can do so through Multi-Safari. Much along the lines of the stand-alone versions of IE that are out there, these packages allow you to run older versions of Safari.
Interestingly enough, though Multi-Safari worked fine under the retail version of Leopard, it stopped working under OS X 10.5.1, offering the error “You cannot use the application ‘Safari 2.0.4’ with this version of Mac OS X.”. Fortunately, the maintainer of the Multi-Safari project, Michel Fortin, looked into it; and, within his blog entry describing the issue, some of the commenters chimed in with their own suggestions.
In particular, Thomas Aylott deduced that “Apple has specifically blocked all versions of Safari < 3 from running on Leopard 10.5.1” (d’oh!), but figured out a workaround by tweaking some of the package internals. To Thomas’ credit, he also compiled a fixed-and-ready-to-download version of Safari 2.0.4. I’ve tried it on my own machine and it works great.