(That's why some of those products have such high gravities, of course: it's not that they're selling well - it's the fact that so many people just buy one copy, each through their own hoplink, so the gravity rises and rises and rises, though the sales numbers may actually be comparatively small, compared with many of the products I'm promoting with gravities around 5 or so, some of which are selling by the thousand. But I digress ...).
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.