That's where PlugSuit from Infinite Labs comes in. PlugSuit is a Leopard compatible plug-in manager that will work with many popular Tiger plug-ins like Inquisitor and SafariStand without changing a thing. Infinite Labs also makes the great free plug-in, Twicetab (that allows users to open up a new tab simply by double clicking on the tab bar, a la FireFox or Opera), and users even have the option of downloading and installing PlugSuit and Twicetab together. Plus, it's free! It's important to note that that plug-ins that were not designed with Leopard in mind may still have some problems or not function correctly. Infinite Labs says that most that were developed for the latest Safari 3 beta should work, but double-check with the plug-in developer to ensure compatibility.
My favorite non plug-in Safari hack, J.Y. Design's Full Screen script, works just fine with Leopard.