To finish an article on the web today is to immediately enter the desperate, and strangely tedious, world of content discovery. Eager to keep visitors on their websites for as long as possible, publishers have long built widgets filled with links for their readers to consider. But increasingly, those links are pointing away from their own sites to titillating items from other publishers, many of whom have something to sell. Readers are clicking, publishers are reaping millions, and the startups that power these modules are profitable and on the cusp of becoming public companies. Over the past few years, paid links like these have popped up on some of the biggest sites of the web: CNN, Fox News, MSNBC, USA Today, and Huffington Post, to name a few.