Firewheel Design has some of the most beautiful royalty-free icon packages available for products. The Manhattan UM collection is just gorgeous. For only US$79 we could really improve the visual appearance of our homegrown web applications. Firewheel is not alone. Others including IconKits and InterfaceLift have great icon collections for both Mac and Windows platforms (check out the stock icons collection). UI Icons and IconExperience also have really nice designs available.
There are some open source icon designers, KDE-Look has a great collection of KDE icons (mostly PNG files). But you have to dig and look to find a collection as clean and complete as offered by the sources mentioned above. The royalty-free icon collections offer so much to developers. Quick, clean professional.