I have collected 5 resources for free PNG icons. They could be used on websites, mobile applications and other stuff.
They are available with multi-dimentions as follow:
These icons are available with PSD file. Free to download.
Monocrome vector icons useful for multi-purpose. The set include actions, media, devices and mobile-specific ones like battery status icons.
3) Seven Icon Pack
set of 475 high-quality, vector and free icons that has items for actions, social networking, devices and more. They come in .SVG + in transparent PNG with 32*32px, 48*48px and 100*100px sizes. And, they are free to be used in both personal and commercial projects.
This website creates a free original icon everyday to be downloadable as .PNG image. Rather than widely used icons, the project focuses on items that we may not find elsewhere.
Icons are mono-colored, in 126*94px size and they are all very detailed.
The set consists of 247 items, all of them designed in the same style and comes as .PNG files. There are 2 sizes in the download package: 32*32px and 128*128px. This hand-drawn icon set has almost all the items a web app. can need from actions to media or devices.