The last two Cool Tool of the Month featured online development environments for modern Web design. This month’s post rounds out the trilogy with CodePen, an HTML, CSS, and JavaScript editor for testing and sharing “pens” with others. The interface is much more slick than JSFiddle or JSBin so it is great for showcasing to clients or collaborating with colleagues.

Check out David Walsh’s interview with Chris Coyier (one of three “authors” along with Tim Sabat and Alex Vazquez).