Waypoints
Waypoints is a library that makes it easy to execute a function whenever you scroll to an element. 
var waypoint = new Waypoint({
element: document.getElementById('thing'),
handler: function(direction) {
alert('You have scrolled to a thing')
}
})
If you're new to Waypoints, check out the Getting Started guide.
Read the full documentation for more details on usage and customization.
Shortcuts
In addition to the normal Waypoints script, extensions exist to make common UI patterns just a little easier to implement:
Contributing
If you want to report a Waypoints bug or contribute code to the library, please read the Contributing Guidelines. If you need help using Waypoints, please do not open an issue. Instead, ask the question on Stack Overflow and tag it with #jquery-waypoints. Be sure to follow the guidelines for asking a good question.
License
Copyright (c) 2011-2014 Caleb Troughton. Licensed under the MIT license.