unDraw Free Illustrations
You’ve more than likely seen unDraw’s work on other sites. There’s no mystery why: Their work is totally free to use, and requires no attribution. And the clincher, of course, is their work is good.
Their main available body of work is all drawn in the same style, and covers a huge variety of subjects. Your keywording skills are going to get some exercise, because the images are not organized in any particular way—you’ll need to use its search function to find just the right image you want. But not only is their art keyword-tagged very well (which means there’s a very good chance you’ll find what you’re looking for even with minimally adequate keywording abilities), the style and image designs are often highly adaptable to a variety of different subjects.
As if that wasn’t enough, here’s a feature that may blow you away: you can specify a color theme; it’ll give you the same artwork in the colors you want. Now you can achieve color objectives you may desire, like have illustrations that match your brand’s color identity, or use color as a way to visually categorize your content. Cool!
Here’s an example, an image they label Diary in three different colors I’ve specified (click to enlarge). You’ll note that these are non-standard Web colors I put to work here; I entered color hex codes to get the precise colors I want.