You could spend an afternoon Googling for them, or you could visit the following sites, which already offer links to numerous great extensions.
Safari Extensions Tumblr blog – Jonas Wisser has set out to list every available Safari extension he comes across. The site lists links and screenshots for the extensions. Generally speaking, this is the first site you’ll want to check.
PimpMySafari – This site has been around for a long time, but with the release of Safari 5 and a new site owner, it’s taken on new life. Unlike the previously mentioned blog, PimpMySafari promises to feature only the best extensions as determined by Scott Jangro. The site also features plugins and other ways to pimp out Safari.
Reddit Extension list – Some dude over at Reddit has composed a list of great extensions. I’m not sure how long the list will remain useful.
Safari Extensions list – Mac OS X Tips (a UK site) has put together a small but useful list as well. Be sure to check this site out for other useful Mac OS X tips!
Apple will soon offer a list on their site of “approved” extensions, but until such time, these sites offer plenty of useful add-ons for Safari users.