Jinriki Vocaloid (人力ボーカロイド, lit. "Manual Vocaloid") is a common practice in Nico Nico Douga targeting Vocaloids, and involves re-editing an existing singing voice (especially a singer or anime character), extracting tones as WAV files and reassembling them. It is different from the popular singing synthesizer Vocaloid, made from YAMAHA.
First Example: Kokoro covered by Utada Hikaru
Response from Utada Hikaru herself