Microsoft Songsmith is music composition software for Windows, launched in early 2009. Songsmith immediately generates a musical accompaniment after a voice is recorded. The user can adjust tempo, genre (such as pop, R&B, hip-hop, rock, jazz, or reggae), and overall mood (e.g. to make it joyful, sad, jazzy, etc.). (via Wikipedia).
Shortly after its release, people started using it to remix popular songs. They take the original vocal track, run it through Songsmith, and produce new musical accompaniment. The results are matched to the original music video, with often comical results. Five such videos produced by Appealing Industries are posted below: