Martin Ankerl offers a very compact but quite usable GTK theme at http://martin.ankerl.com/2007/11/04/clearlooks-compact-gnome-theme/. It's based on the default GTK Clearlooks theme, so it's visibly unobtrusive, just smaller.
It will make all GTK apps, including Firefox, Thunderbird, and Eclipse, more compact.
To install it, first install the gnome-control-center if you haven't already done so.
sudo gnome-control-center from any teminal window, and click “Theme”, then click the “Install New Theme” button. In the theme location editbox, copy in “http://martin.ankerl.com/files/ClearlooksCompact.tar.bz2”.
This only installs the theme; to use it, select the highlighted theme, then click the “Theme Details” button, then select “Clearlooks Compact” in the “Controls” list. Then Close “Theme Details”, and “Themes”.