Web of software interface designers are in constant need of free png icons they can use in their project. I have found some great websites offering free icons for download and I am publishing list of best ones for my personal reference, hoping other people will find it helpful.

1. Icon Archive

Huge archive of icons for Windows and Mac, also available in png format. All icons are contributed by various icon designers all over the world. everything organized and sorted in categories.

2. Free Icons Web

Another extensive collection of user contributed icons. Only some icons are available in png format, but maybe you want to check gif icons page

3. Feed icons

Rss feed icons to use in various projects. Very helpful for Wordpress skins. Various colors available.

4. Mayosoft

Software related icons.

5. Iconizer

Few hardware icons.

6. Afterglow

Some great characters-two sets available, both in png format.

These are only few picks from tones of icons available online.

For more extensive resources you might want to check these lists of free icons:

1. Maxpower

2. TutorialBlog-free stock icons roundup

3. Smashing magazine-icons, buttons and templates

4. Wikipedia: icons

Looks like you are going to download lots of stuff. Enjoy.