If you are a web designer or software developer, you definitely would require icons. Now you have three choices, either make them yourself (which might not be a good idea if you are not into graphics) or download those available for free on the net and finally to buy them from professional graphics designers.
The second option might be Ok if you are not looking for high quality icons. However, if you are a software developer of commercial applications, you should take a look at Iconplant.com. Their XP style icons are really professional looking and adher to Microsoft's style guidelines. Even if you don't find something you like from their vast list, for example, say you are looking for custom made Vista Icons, you are in luck. Iconplant.com even offers full icon customization services. Their normal production schedule is five icons per day but they are ready to work with you to meet your deadlines.
Icons are provided in three formats, namely, gif, ico, and png with each purchase. Vector icons are also available on request. The prices are quite reasonable for the quality of work.
Of course, as regular readers of my blog are aware, I won't be putting up a link to a site unless there is something free. Well, you can download a zipped file of free icon samples from their site to test out.