Robin Good: Tagxedo is a great, free web-based tool that allows you to create stunning covers, images for articles or posters, based exclusively on words.
You can either input the words yourself, or provide a website URL, a Twitter account, a news or web search and Tagxedo will create a "word cloud" by tapping into that word "universe".
There dozens of different controls to customize your word-art creations including the ability to change layout, fonts, colors, shapes and even density of your artwork.
The final work can be shared easily on social media or saved in your preferred graphic file format (jpg or png) and at your desired resolution.
You can see some examples here: http://www.tagxedo.com/gallery.html
or try it out immediately here: http://www.tagxedo.com/app.html
More info: http://www.tagxedo.com/