This list is now more or less superceded by https://extensions.gnome.org/ where extensions can be one click installed and configured.
There are several addons for gnome-shell. Please help me compile a list of existiong addons
I have answered my question by adding the extensions I know of as answers. Since there are many extensions each single answers is still a stub.
Here's a video of some of the extensions in action:
Now there's the alpha of extensions.gnome.org where you can get some more extensions. Hopefully in the future this will make this community wiki obsolete :P
alternative-status-menu
For those who want a power off item visible at all the time.
user-theme
Loads a shell theme
weather extension
shows the weather right next to the clock.
For installation instrucions check here
windowsNavigator
Allow keyboard selection of windows and workspaces in overlay mode
I believe that this is the most exhaustive list: http://live.gnome.org/GnomeShell/Extensions
If you're feeling adventurous, you could always try to create your own. Useful guide here: http://blog.fpmurphy.com/2011/04/gnome-3-shell-extensions.html
drive-menu
A status menu for accessing and unmounting removable devices
Intergrate google calendar into gnome-shell
http://www.webupd8.org/2011/09/google-calendar-gnome-shell-integration.html
Still buggy but has to be mentioned: The Mint GNOME3 Shell Extensions (MGSE). It's actually many different plugins like a xrandr extension, a new bottom-panel, a shutdown-menu etc. Those can be installed seperately see here (also from the webupd8 PPA).
Thanks dude for creating and maintaining the post. Useful for everyone :)
Few more
Media player extension
http://www.webupd8.org/2011/11/try-new-mint-gnome-shell-extensions.html
Pidgin Integration
http://www.webupd8.org/2011/10/native-gnome-shell-notifications-for.html
Thunderbird extension
https://github.com/tanwald/gnome-shell-extension-thunderbird-integration
Workspace Switcher Extension,Alternative Status Menu GNOME Shell Extension,SystemMonitor GNOME Shell Extension,Apps Menu GNOME Shell Extension,Places Menu GNOME Shell Extension,User-Theme GNOME Shell Extension,Alternate Tab GNOME Shell Extension
http://www.techdrivein.com/2011/10/7-best-gnome-shell-extensions-install.html
Many standard extensions can be easily installed thr ppa's
http://www.webupd8.org/2011/10/official-gnome-shell-extensions.html
I am sure some day we will have a chrome web store sort of thing for gnome shell extensions too , until then we have to live maintaining a list like these :P (or some one might have already created a better blog having the list)