"Hover Bars" are my first set of GNOME panels and my first real contribution to the Linux community.
A simple mirrored gradient whipped up in Gimp, they are available here in various sizes and levels of transparencies.
The files are named by transparency (%) first, followed by size (in pixels) second.
So "HB-10024" for instance, is a Hover Bar with no transparency and is the default Ubuntu GNOME panel size of 24px.
Whereas "HB-2532" is almost completely transparent and is ideal for larger 32px size GNOME panels.
To install simply drag these images over the panel you wish to change or right click on the panel and browse to the image (Properties>Background>Background Image) .