Gnome 3 convinced me to ditch my 10 year old configuration of FVWM. I really liked how I could have a clutter-free and efficient desktop, but the FVWM configuration file was rather and dual-screen issues, so I switched to Gnome 3.
While I generally like Gnome 3 and how it removes most of the screen clutter, my computer screen is wide, but not very high. I used the "autohidetopbar" extension by fpmurphy. However, I kept re-enabling the top bar by mistake, as my Firefox window occupies most of the vertical space on my screen, so when I would go to click on the Firefox address bar, I would end up re-enabling the top bar. Then the extension stopped being maintained.
So I created a new extension based on autohidetopbar, called... hidetopbar. It hides the top bar all the time, except in overview mode (mouse in top-left corner).
Update: 2013-01-06: I published my extension on the official Gnome extensions site. It was more for practical reasons, and didn't expect to actually have people using it, but have since received a lot of feedback and good patches. A fun experience.