This is a place where you can learn relative pitch.

Relative pitch is the skill that lets you play things by ear.

If you're just starting out: select Level 1 and press "Start". Then press random buttons on the piano keyboard. You'll catch on.
(make sure the volume is up)
This page also serves to demonstrate the methodology used in Ear Sharpener: the iPhone/iPad app.

This page, as awesome as it is, lacks many of the features available in Ear Sharpener. Here is a chart which shows that to you.

This Page Ear Sharpener
Works in Chrome, Firefox, Safari and Opera on your computer Yes! No, Ear Sharpener really fails here :(
Works in Chrome, Firefox, Safari and Opera on your phone Sort of but it's slow and clunky Ear Sharpener won't do that either
Works as a native app on your iPhone No Ear Sharpener excels in this area
Can set the key of the intro No Yes
Has a range of many octaves Just one octave Yes
Can test multiple simultaneous notes Up to 2 Up to 11
Turn off the keyboard sounds No Yes
Has hundreds of levels that increase in baby steps No, only 8 levels Yes