You can get the latest binary (i386) and/or source RPMs (and the SPEC file) of the OpenSolaris’ GTK+ engine Nimbus (Version 0.0.16, source) for Fedora 9 here. If you want to build the source or the source RPM on your own, you’d need gtk2-devel, automake, autoconf, intltool (>= 0.23), gnome-common (>= 1.2.4), icon-naming-utils (>= 0.8.1).
Here is a screenshot of what Nimbus makes your desktop look like:
Edit: Updated to 0.0.17. Find the relevant files here.