WORLD is free software for high-quality speech analysis, manipulation and synthesis. It can estimate Fundamental frequency (F0), aperiodicity and spectral envelope and also generate the speech like input speech with only estimated parameters. It is based on the Vocoder and is used as the base for different UTAU resamplers.
WORLDLINE-R is OpenUTAUs default resampler and wavtool, based on WORLD.
vConnect-STAND is a complete synthesis engine, based on an old version of WORLD. It is mostly used with jcadencii, an old pianoroll editor.
Sekai is a fork of WORLD with many interesting features.