The GTK+ API offers the notion of "stock icons", which is basically
a set of icons (provided by each theme) that are common to a
majority of applications (basic things like OPEN, NEW, ZOOM_IN, etc):
http://developer.gnome.org/doc/API/2.0/gtk/gtk-Stock-Items.html

In addition to simply providing stock icons, GTK goes one step
further and provides something called GtkStockItem, which
includes a default label, mnemonic, and accelerator:
http://developer.gnome.org/doc/API/2.0/gtk/gtk-Stock-Items.html#GtkStockItem

The GtkIconTheme class provides a way to get a certain icon
at a particular size (also allows for some control over
whether it comes from an SVG file, but clearly this class
ain't a paragon of design):
http://developer.gnome.org/doc/API/2.0/gtk/GtkIconTheme.html

If you want a "stock" button (or toolbar button, or menu item)
you can use a method like gtk_button_new_from_stock(), which
will give you a new GtkButton complete with the appropriately
themed icon, localized text, and mnemonic:
http://developer.gnome.org/doc/API/2.0/gtk/GtkButton.html#gtk-button-new-from-stock
http://developer.gnome.org/doc/API/2.0/gtk/GtkToolButton.html#gtk-tool-button-new-from-stock
http://developer.gnome.org/doc/API/2.0/gtk/GtkImageMenuItem.html#gtk-image-menu-item-new-from-stock

It seems that GTK developers are quite comfortable using these
APIs whenever possible; all of the apps I've played around
with on Ubuntu have a consistent look because they're all
picking up the same stock icons/text/mnemonics. Not
surprisingly, Swing apps look very much out of place because
they can't use that pool of stock widgets.

In an ideal world, Swing would have a similar API. Each L&F
could determine whether stock icons are used in buttons at
all (e.g. for GTK and maybe Ocean this would be true, for
Windows and Aqua this would be false). The downside of
course is that if an app wants to look good ("fit in") on
GTK, they'd have to update their code to use the new stock
API in JDK 7, but no matter what, developers would have to
update their apps anyway. The upside is that it would offer
a pretty easy solution for developers who want their apps to
look more native; better to give them an easy approach, because
otherwise it's clear that Joe Developer ain't doing it himself the hard way.

Note that this isn't just about the GTK L&F. The Windows L&F
should be able to provide access to at least some common icons
via the uxTheme APIs. It is expected that Apple will provide
access to a set of common icons in their upcoming Leopard release,
but details are sketchy. We (Sun) should provide a set of
high-quality icons that can be used with Metal/Ocean, or as a
backup for L&Fs that aren't yet updated to provide ones of
their own. (This last idea would be really nice because most
developers have to scour the web looking for free icons, or spend
lots of time implementating their own.)