Despite their diminutive size, icons take a lot of time and effort to design. The canny professional website designer may occassionally use high-quality free icons for some parts of a website. Or even just as holding images until the real icons are produced.
Here are 17 icons sets or icon posts. (A particularly good resource for free icons is Smashing Magazine.)
Note: some of these icons do require payment for commercial use. You can also purchase some sets for exclusive use.
1) Noupe's handy post on the the 50 most beautiful icon sets from 2008.
2) Sweet food-related icons (including cakes and pastries).
3) Smashing Magazine's content management system icons. Useful for functions like billing, admin, save etc.
4) DeviantArt, a site filled with quirky user-submitted art, has quite a lot of icon sets. Many are licensed under creative commons non-commercial use licenses, so you may not use them on business sites. However, you can negotiate with the artist if you like what you see.
5) ...The same applies to Flickr. Perform a search on icons and you'll get an eclectic mix of strange imagery and nicely designed web art. You'll need to check the licensing on each icon.
6) DryIcons has a load of free icons. However, you'll need to pay a small amount for business use, and a little extra for exclusive commercial rights.
7) Icons for social media, browser, ecommerce and more at Smashing Magazine.
8) Richworks' leather-clad social media icons. You can almost smell them.
9) Lots of credit card, cash and financial icons from Chalkworks.
10) Clean, simple white-on-black icons for a variety of web tasks.
11) Work in medical or health-related field? Here's a set of a medical icons. All the syringes, stethoscopes and scalpels you'll ever need...
12) Hand-drawn sketch style icons including shopping cart, Twitter and RSS icons.
13) Apple's official icon download page. Lots of Apple product icons, including iPad and iPhone, as well as Mac-style icons for many more web functions.
14) Stacks of credit card and Paypal icons with code ready to copy and paste.
15) Plain, functional but neat set of web icons (including Home and social media icons).
16) Green, rounded, plump little ecommerce icons from Iconeden.
17) The Iconspedia blog has a good selection of classic ecommerce icons, including most major credit card icons.