For those of you who are interested in being able to poke around with a piano, but never seem to be in front of one, here’s a simple midi keyboard application I’ve made (Mac OS X only for now), aptly named Midi Keyboard 1.0. It’s free!
The main feature of this app is the ability to use your computer keyboard (commonly called the ‘QWERTY keyboard’) as a piano keyboard. This is a feature that now comes standard with most DAWs (digital audio workstations) like Garageband, Logic, Sonar, etc., but for most beginning musicians, those programs are either too expensive or too advanced.

QWERTY keybaord meets piano keyboard
This application is designed to help a beginning musician get acquainted with pitches, intervals, and note names quickly and easily. Once you hit the caps lock button and start playing around, the program automatically tells you what note letter name you are playing (C and G in the picture below), what interval the last two notes you play formed, and how many semitones that interval is.

Screenshot of the interface.
Additionally, if you choose a key the program will tell you what solfége syllable your note is (using the movable do system).
This is in a beta stage and is likely to be part of a larger project involving interval recognition. But in the meantime, you can download the latest version here:
Midi Keyboard 1.0