Well, according to widget : widget's Terms and Conditions page, their icons are all available under Creative Commons noncommercial and/or commercial licenses.
Icons with the noncommercial license are free for noncommercial use, including noncommercial websites. I believe this would qualify as a noncommercial website.
Icons for commercial use cost $9.99 a year for the license.
From their T&C page, it sounds like *ALL* of their icons are available for noncommercial use, with some also available for commercial use.
According to the badge on their icon archive page, but the Magnum Opus set is non-commercial-only. Which means we're free to use it on here, as long as we give them proper credit, aren't charging any money anywhere, and don't modify the icons.