Synthesizer V is described as 'vocal synthesizer developed by Kanru Hua, aiming for "artistic perfection of artificial voices"'. There are eight alternatives to Synthesizer V for Windows, Mac, Linux, Online / Web-based and iPad. The best alternative is VOCALOID. It's not free, so if you're looking for a free alternative, you could try DeepVocal or UTAU. Other great apps like Synthesizer V are Emvoice (Paid), Alter/Ego (Free), Chipspeech (Paid) and eCantorix (Free, Open Source).
Alter/Ego is based on the award winning technology featured in chipspeech but instead of targeting Vintage voice technology, it focuses on more ‘modern’ (1990+) singing synthesis algorithms and research.
eCantorix is a singing synthesis frontend for espeak. It works by using espeak to generate raw speech samples, then adjusting their pitch and length and finally creating a LMMS project file referencing the samples in sync to the input file.
Renoid is based on the idea of Sato-san, the Renoise-Gate webmaster. Renoid is a virtual singer system using Renoise XRNI or SoundFont SF2 sound library and cascaded pitch-corrector. Through this, virtual singing can be used without any special program.