I found something wonderful. Adium is a popular (or the popular) instant messaging client for MacOS. Various people have produced addon icons and sound files and so on for it, and they are available on the AdiumExtras site.
Now, one or two of the items in each category are generally beautifully done. They're not the interesting ones. You'd be far better advised to take a look at the many, many shockingly ugly ones, and read the comments, and the attempts at defending the horrible icons (or whatever) by their designers. Great fun.