On a prominent news site (sry, can't remember exactly which one) I saw a really cool effect... when you scroll near the bottom of the page, a new element slides down from the top of the browser viewport with a lot of social media options (tweet, share on facebook, etc). I'd like to emulate something somewhat similar... in fact, there's really a ton of things I could think of to do if I knew how to trigger a function when the user is near the bottom of the page...

So, my question is very basic: how does one trigger a function when the user has scrolled near the bottom of a dynamically sized page?