"Autohide Top Bar" is a new GNOME Shell extension created by fpmurphy to autohide the GNOME Shell Top Bar (panel) but it doen't just provide a simple autohide feature - the extension also lets you double click the Top Bar to toggle hiding on / off.
If double clicking to toggle autohiding on / off doesn't work properly, open the "extension.js" file and increase the "const TIME_DELTA" value (something around 2000 - 2500 should do it).
Since I can't make a screenshot of an autohiding Top Bar (GNOME Shell panel), here's a short video demoing this extension:
To install the extension, extract the downloaded archive and copy the "email@example.com" folder to ~/.local/share/gnome-shell/extensions/, then press ALT + F2 and enter "r" to reload GNOME Shell.
Download GNOME Shell Autohide Top Bar extension (look for "autohidetopbar-1.0.tar.gz").
Update: for GNOME Shell 3.2, see: Autohide Top Bar Extension Finally Available For GNOME Shell 3.2
For even more screen real-estate, see: How To Remove Maximized Windows Titlebar In GNOME Shell