The Extensions Gallery is essentially a center for add-on features and toolbars for a variety of different web programs and services. Ones that are most likely to be popular among Safari users include extensions for Twitter, The New York Times, MLB.com, and Amazon. There's even one called "Better Facebook" that will actually tell you when you've been "unfriended." Bummer.
Then there are a few that stood out a bit, like fixing an issue that "prevented boarding passes from www.aa.com from printing correctly," and another that "fixes an issue that could cause Flash content to overlap with other content on www.facebook.com, www.crateandbarrel.com, and other sites when using Flash 10.1." Go figure.
Safari 5.0.1 is available for downloading now.