GNOME Tweak Tool got extension installation support for a while now (GIT), but with the release of version 3.0.5, this is finally considered stable enough.
To install an extension, download it (via GIT or whatever), zip it and use GNOME Tweak Tool to install it (option available on the "Shell Extensions" tab). In the same way you can also update the extensions:
(screenshot thanks to rAX)
The latest GNOME Tweak Tool 3.0.5 is already available for Arch Linux, but it hasn't been packaged for Ubuntu (either in the GNOME 3 PPA or Oneiric) or Fedora yet so if you want to use the latest version, install "git-core" and run the command below to get GNOME Tweak Tool from GIT:
cd && git clone git://git.gnome.org/gnome-tweak-tool
Then simply double click the "gnome-tweak-tool" file under the gnome-tweak-tool folder in your home directory.
Note: GNOME Tweak Tool currently requires GNOME Shell to be installed and will crash if Shell is not installed.
Thanks to rAX for the tip and second screenshot!