You can create a custom icon by copying any image from a web browser's window [with OmniWeb: Right Click:Copy image to clipboard], and pasting it into the Finder's Show Info window of any file.
Cool.
[Editor's note: This trick is old-hat to many Mac users, but it's probably new to a number of readers here. The expansion of this, of course, is that any image on your clipboard can be pasted to create a custom icon.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020320231307879