Hi,
I was createing maps/mods for Warcraft III for about 2 years.
I remember there was a good page to download icons/models/skinns for Warcraft III. But you can also export them and use them in your games!
Take a look at
http://www.hiveworkshop.com/forums/icons.php
So, what do you need to do?
Download the BLP exporter: http://dl.dropbox.com/u/8537768/Uploads/blp.zip
Then you can download all resources from this site. Open them with the PLB lab and export them to tga, bmp or whatever you want.
You can also download models from this site and import them with milkshape. But you will miss some textures, because you haven't Warcraft installed.
If you want to know how this resources are licensed, look in the readme.txt that come with them.

An icon example:

A skin example:

Thank you, I might be able to use some of them. However, many readme.txt files say:

Please give me credit for my work
Do not redistribute this icon without consent!

so what does that mean for me? do i have to contact the auther if i want to use it in my game and ask for permission? or does "redistribute" mean that I'm simply not allowed to upload it to any other icon collection?

All these icons are usermade.
I think you shouldn't upload it again to any other website.
But maybe you could send a mail to one artist and ask what does this mean.
It would be helpful for all of us.
The only think you shouldn't forget is to give credits.