FreePiano is a opensource software that can let you play music with your computer keyboard.

Features of FreePiano:

• Completely free, you do not need to purchase to use all the features.
• Using VSTi, you don't need to install a virtual MIDI device.
• Support a variety of audio output, including DirectSound, WASAPI and ASIO.
• You can define any key on the keyboard and display functions.
• Multi sets of keyboard layout can be switched anytime during play.
• Export your song to mp4 directly.