A marquee feature of Safari 5 is its ability to use 3rd-party extensions. Apple is collecting examples of great ones from registered developers to be featured in a gallery. There's no word on Apple's judging criteria or when the gallery will be launched, but Apple is really pushing this technology so be assured that it's going to get a publicity push.
It's also unclear just how Apple will distribute the extensions. A blurb inviting submissions reads "The Safari Extensions Gallery will feature a one-click installation process that allows users to seamlessly install extensions from your website without ever having to leave the Gallery." That sounds like developers will host the extensions while Apple provides the link. Some serious vetting will have to ensure quality.
While we wait for Apple's gallery, check out the collection at Safari Extensions. It's a nice collection that's growing all the time.