This instructable shows you how to create a very simple Arduino-based sequencer with nice features:

Multiple synthesizer projects has been done for the Arduino, but few has been able to utilize the full power of the Arduino processor. DZL from GeekPhysical wrote a 4 voice wavetable synthesizer that is one of more advanced software based synths for the Arduino.  It has wavetables included (sine, saw, square and triangle) and envelopes to create beats.

Implementation instructions can be found here, while the Arduino code can be obtained from GitHub.

[Via: Instructables]