For everyone using Gnome 3 (the awesomest DE!) they have launched a pretty wicked shell extension site, it’s still in alpha but worked great for me.
You can create an account, write comments and even upload your own extension, but the main thing is they feature a one-click installer for existing extensions, just hit that link and the selected extension is installed and even loaded directly, no download/install/restart crap. It will also detect currently installed extensions, this thing is just awesome! One thing I miss though, is the ability to search for extensions, but I am sure they will fix this:) Check it out here.