The folks at web design from scratch have recently posted a list of their 10 favorite web site designs. The sites make up good sources of inspiration. I particularly like the IconBuffet. For about $300 you can get a great set of icons that will make your application stand out.
Another source to consider is the icons we added to the 2007 templates. We spent fair amount of time coming up with different concepts and also improving the icons in the picture wizard.