As far as skins go, most can be downloaded from the SVN. It's an excellent system as you can get up-to-the-minute updates whenever you want them, and an SVN client only downloads those files that have changed since the last version.
For XBMC Scripts, we have, well...
XBMCScripts.Com.
And there's also a site somewhere which has a ton of wide icons. MediaIcons or something like that, never payed it much attention myself...
Even if XBMC development were to stop today, it would be years before people stopped downloading it, perhaps decades before it fell out of use entirely. We're talking award winning software here!