Icons are a huge part of your design choices, so it is critical to have them be both beautiful individually and uniform in style across your application.
Recently I was working on Raw Sensor Data and needed to find a set of icons that I could use for the categories of sensors. After checking my usual sites for creative commons clipart (www.clker.com being my favorite), I was frustrated to find not only a serious lack of icons that suited my needs (Finding gyro and XYZ axis icons were particularly difficult!), but also generally poor quality and many sites.
Right when my frustration was skyrocketing I discovered The Noun Project.
The Noun Project has a truly staggering number of beautiful, well designed, icons available covered under Creative Commons, meaning they are free for use as long as you attribute them. You can do this by downloading a version with attribution attached (as below) or by adding it into your app, store listing, website, etc. Alternatively, for a monthly fee you can get royalty free images from The Noun Project.
Hands down, the strongest part of The Noun Project is the high level of design and consistency between the icons. You can take a look at Raw Sensor Data to see what I mean. For the vast majority of other Creative Commons sites, you are going to spend a lot of time tweaking the images to get them looking the way that you want them. With TNP, many artists will create collections of stylistically consistent icons to ensure that everything looks perfect and works will together.
If what they have on hand doesn't suit your needs, they have a cloud-connected visual asset library called Lingo which is super powerful and ensures that your icons will be consistent throughout your applications.
