We are looking for an icon library that is open licensed. We want to use the icons as a prototype not the final version of the application, but they need to be open to use.
EDIT 1: Even though many of the responses are good and useful, I will mark as correct answer the one with more votes. First because I like it, and second because the amount of votes. Thanks to all of you for your responses.
I don't know of any standard library, but I've found that http://iconfinder.net is a useful source of icons. License information is included with each, and you can view and download entire sets of icons under the same license.
I tend to use Sanscons when I need some icons during development. They are free, simple, and let you change the color easily via CSS. There is a constrained set, so I do not spend too long hunting for the best one, but I can usually find a suitable match pretty quickly.
Because they are all the same color, they tend to blend in to the design unobtrusively.
The Tango project produces an icon library that is used in a number of open source projects. See: http://tango.freedesktop.org/Tango_Icon_Library.
also, I don't know of any standard library.
but i prefer famfamfam's. You can find famfamfam's icon repository here : http://www.famfamfam.com/lab/icons/
I think famfamfam are great, but... I found the Iconza site the other day, and have been impressed with how easy it is to create a set of custom colored icons.
If you're looking for iPhone icons, http://glyphish.com/ is great
We just went through a similar decision at my company. Ended up using the Fugue icon set from Pinvoke.
It's free if you give credit, $50 otherwise.