Adam Whitcroft has released a 300 icon kit for web and user interface design called Batch. Each icon is a single Photoshop layer making blend modes and scaling a breeze.
According to the license, WordPress theme or plugin authors should be able to use these in their works:
You are free to use Batch (the “icon set”) or any part thereof (the “icons”) in any personal, open-source or commercial work without obligation of payment (monetary or otherwise) or attribution. Do not sell the icon set, host the icon set or rent the icon set (either in existing or modified form).
While attribution is optional, it is always appreciated.
Intellectual property rights are not transferred with the download of the icons.
This icon set previously had a price tag attached to it. Adam explained why that’s not the case anymore.
Unfortunately, that license is not GPL-Compatible, and so the icon set cannot be used in any themes or plugins hosted for free on WordPress.org.
“Do not sell the icon set, host the icon set or rent the icon set (either in existing or modified form).” is not a GPL-Compatible statement. Freedom means allowing other people to do things with the creative work too, even things you might not like.