I'm mid-way through a Silverlight application I'm writing called "MossyVideo Player". In the design of my application, I am making use of Icons to represent navigational elements that whilst aren't 100% relevant to the task, they are however close.
I could of designed the icons from scratch, but in truth I don't have the time or patience to do this, so in light of this I began an online hunt to find out what Icons are available today that are worth buying.
I've begun to compile a list of Icon providers I've found, but would love others to ping me with their favorite source for these little nuggets of brain candy (which I'll then post online somewhere with summary of each provider).
Icons can make the difference in my opinion on whether a user interface is visually appealing or absolute rubbish. I'm sure their is a detailed brain dump on why they exist and how important they are from a user experience perspective (I'll also dig up some more detailed wording on why, where and how they should be used from some of our UXE folks when i'm next in the room with one of them).
I also think that if you have the design ability and know how, you could make some nice $change in selling them (royalty free) to the next generation of RIA designers.