Source: http://lifehacker.com/5558096/safari-extensions-catalogs-add+ons-for-safari
Apple won't be opening their extension gallery for Safari until later this summer. If you want to play with extensions sooner rather than later Safari Extensions is an unofficial catalog of available extensions.
The current offerings are small compared to the massive catalogs for browsers like Firefox and Chrome but with Safari 5 hot off the development press this morning we're not in any position to complain—especially in light of the fact that in the time it took to write this article the number of offerings doubled! Check out the extensions they have or submit extensions you've found in your travels. Safari Extensions is a free site and requires no registration. Thanks jwisser!