UTAU is the grandfather of free vocal synths. It works similarly to Vocaloid (phoneme-based input). The power? Tens of thousands of free voicebanks created by fans. You want a raspy rock voice? A sweet anime girl? A deep male opera singer? It exists.
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Recommended Free UTAU Voicebanks: Teto Kasane (the queen of UTAU), Ruko Yokune, Matsudappoiyo.
This isn't free software, but it is a common source of confusion. Piapro Studio comes bundled with certain DAWs (like PreSonus Studio One). If you bought a cheap audio interface that included Studio One, you might already own a "free" vocal synth. Check your software bundles. vocaloid voicebank free
Technically, UTAU is a separate program. However, because the workflow is identical (placing notes and lyrics), many producers use UTAU voicebanks while saving up for Vocaloid. The quality of modern UTAU banks (like Ruko Yokune or Kasane Teto – yes, the internet meme) is incredible.
Here is my exact recommendation for someone who wants to start making Vocaloid-style music for $0: UTAU is the grandfather of free vocal synths
After you master that, you can explore UTAU/OpenUTAU for more anime-style voices, or save up for a full Vocaloid voicebank (like Hatsune Miku V4X or Megurine Luka V4X).