Since the birth of UTAU, resamplers have been crucial to ensure vocals can be synthesized. There have been 13 resamplers that have been developed that we know of. Resamplers are an intergral piece of UTAU, with many being developed by people not involved with the development of UTAU itself.
What are Resamplers
Resamplers are what's responsible applying formant filters, generating .frq files (or whatever filetype the resampler uses) to speed up rendering, and essentially make the UTAU sing.
Selecting Resamplers
To select a resampler, navigate to Project, then click Project Property. At the bottom of the window will be a box for resamplers labeled Tool 2 (resample).
Either type out the resampler's name in the box, or click "..." to navigate to the resampler.
List of Resamplers
Although these are samples of each resampler, keep in mind that resamplers can sound drastically different depending on the voicebank, for better or for worse.
resampler
Default resampler that comes with UTAU. Uses frq files. Latest versions are always packaged with the program. Developed by Ameya.