Max Anderson has an interesting article describing tips on how to make Eclipse look better on Linux.
As he demonstrates via screenshots, Eclipse under GTK and Linux uses by default a larger font and much more space between elements.
Systemwide GTK+ settings
By shrinking the font sizes and the spacing between the elements, you can make the Eclipse layout a lot more compact and hopefully more usable. One of the commenters suggests a .gtkrc-2.0 change which applies a set of the font and spacing changes.
One of the drawbacks to applying these changes is that all GTK+ applications will inherit these changes which does make some applications look quite funny. After trying out the suggested .gtkrc-2.0 changes for a few hours, I reverted my configuration to the default configuration.
Eclipse specific settings
There is another option to increase the information displayed on a screen in Eclipse. By changing the default fonts used in Eclipse, the display can fit a lot more items (classes, files, code) but to me it appears out of proportion with the default spacing being used. Another problem with this approach is that there doesn’t appear to be any method for specifying the font used for the Project Explorer (except changing system wide defaults as mentioned above) which make the application appear quite odd.
Hopefully Eclipse 3.4 will make some progress in making the GTK version of Eclipse first class.