Well I don't know about other collections, but I don't think there's much to prevent you from using the
Java Look and Feel Graphics Repository. Unless you need to edit the graphics, or plan to use them to disparage Sun.
Sun grants you ("Licensee") a non-exclusive, royalty free, license to use, and redistribute this software graphics artwork, as individual graphics or as a collection, as part of software code or programs that you develop, provided that i) this copyright notice and license accompany the software graphics artwork; and ii) you do not utilize the software graphics artwork in a manner which is disparaging to Sun. Unless enforcement is prohibited by applicable law, you may not modify the graphics, and must use them true to color and unmodified in every way.
It's easy enough to include a copy of the licence in the same directory as you put your images (probably in a jar file). Perhaps your application's "About" page could have a link to the graphics license, to be safe. I don't
think there's much more to worry about. Sun wants to promote their look and feel; there's little motivation for them to give people crap about using their graphics which they distribute for precisely this purpose. Of course if you want graphics which are more compatible for a different look & feel, like say Microsoft,
you should probably look somewhere else...
(I'm curious to see what repositories may be suggested by others.)