http://florence.sourceforge.net
Florence is an extensible scalable virtual keyboard for GNOME. You need it if you can't use a real keyboard either because of a handicap, disease, broken keyboard or tablet PC but you can use a pointing device. If you can't use a pointing device, there is gok: http://www.gok.ca/
Florence stays out of your way when you don't need it.
It appears on the screen only when you need it.
There is an auto-click functionality To help people having difficulties to use the click button.