Apple's brand-spanking-new Safari 5 release has support for extensions baked in, but the expected web gallery of extensions isn't quite open yet. Still, enthusiasts are likely to start baking up homebrew extensions, so here's how to make extensions possible.
If you want to get in early on the action, follow the steps detailed by the Tech-Recipes' blog to enable the Extensions menu. Basically you'll be enabling the Develop menu from the Advanced preferences section, then clicking "Enable Extensions" in the Develop menu that appears. From there you'll now have an Extensions menu in your preferences, and Safari will accept extensions or at least those early work-ups that are popping up (with .safariextz file names).
Safari 5: Enable Extensions Menu [Tech-Recipes]