I found this compilation on Discord and asked for permission to include it here. Since it didn't have any descriptions, I decided to add them. When you open each plugin, you'll see the creator's information.
THIS IS STILL IN WIP, MORE DESCRIPTIONS ARE MISSING.
in the zip are 54 plugins, plus 2 for MAC
Credits to @sourmiku
2bancopy: Apparently, this is another plugin for making note copies.
AddOverlap: A tool that makes it easy to add Overlap to notes.
AddPortamento: The same as AddOverlap, but with Portamento.
Alpha3-r2 (Hanakago): Allows you to add kanji to notes.
AutoCVVC: This is a tool that converts USTs to CVVC. It's very useful in traditional UTAU, and useful in OpenUTAU if the reclist doesn't have a phonemizer. The archive contains two versions of the plugin. We recommend "autoCVVC2.0-0.06.zip."
AutoPitchWriter: Adds automatic tuning to selected notes.
back2cv: a VCV to CV converter
betins-121124: improves a significant number of UTAU features
bkh01_055: adds flags and various other features to the selected voice bank (ONLY WORKS WITH MONOPITCH) (IT IS RECOMMENDED TO BACK UP THE VOICEBANK)
clipboard: two plugins that improve the copy and paste function
convertvb2romaji: there are three versions of this plugin; I wouldn't know which one to recommend, but in short, they convert voice banks to Romaji. (oto.ini may not work properly after this)
envedit163: Improves the cross-fade function
FilestoHiragana: Changes romaji voicebank files to hiragana (oto.ini may not work properly after this)
flagedit113: Significantly improves the "flags" option, allowing you to choose which flags are used for each specific note.
freqtracer242_2 and frqeditor20160417: Improves the frequency editing options; I consider these very useful.
Gakuya: Helps create readme.txt, character.txt, and install.txt files, and removes unnecessary files from the voicebank. It also notifies you if any are missing.
HNP_PreNoteFit_v001: It's complicated to describe what this plugin does, but it basically improves the pre-note functionality.
Humanizer: Helps achieve a human/realistic result when tuning.
hyphen: I haven't been able to find exactly what this does.
resampler_patcher1.4: Makes resamplers run faster, recommended for resamplers like wn4u (ONLY WORKS ON "CLASSIC" RESAMPLERS, WILL NOT PATCH RESAMPLERS LIKE HIFISAMPLER)
iroiro2: The best plugin for classic UTAU. It's basically a Swiss Army knife, with an insane amount of features.
JapaneseLyricConverterForVocalSynths_v120: Converts romaji to hiragana on a wide range of synthesizers.
Kaoemoji: This plugin is useless but cute; it allows you to add notes with kaoemojis.
kbf110731: Improves several note-related features
longrcutter_1.01: Splits long rests, which I find quite useless
THIS IS STILL IN WIP, MORE DESCRIPTIONS ARE MISSING.
in the zip are 54 plugins, plus 2 for MAC
Credits to @sourmiku
2bancopy: Apparently, this is another plugin for making note copies.
AddOverlap: A tool that makes it easy to add Overlap to notes.
AddPortamento: The same as AddOverlap, but with Portamento.
Alpha3-r2 (Hanakago): Allows you to add kanji to notes.
AutoCVVC: This is a tool that converts USTs to CVVC. It's very useful in traditional UTAU, and useful in OpenUTAU if the reclist doesn't have a phonemizer. The archive contains two versions of the plugin. We recommend "autoCVVC2.0-0.06.zip."
AutoPitchWriter: Adds automatic tuning to selected notes.
back2cv: a VCV to CV converter
betins-121124: improves a significant number of UTAU features
bkh01_055: adds flags and various other features to the selected voice bank (ONLY WORKS WITH MONOPITCH) (IT IS RECOMMENDED TO BACK UP THE VOICEBANK)
clipboard: two plugins that improve the copy and paste function
convertvb2romaji: there are three versions of this plugin; I wouldn't know which one to recommend, but in short, they convert voice banks to Romaji. (oto.ini may not work properly after this)
envedit163: Improves the cross-fade function
FilestoHiragana: Changes romaji voicebank files to hiragana (oto.ini may not work properly after this)
flagedit113: Significantly improves the "flags" option, allowing you to choose which flags are used for each specific note.
freqtracer242_2 and frqeditor20160417: Improves the frequency editing options; I consider these very useful.
Gakuya: Helps create readme.txt, character.txt, and install.txt files, and removes unnecessary files from the voicebank. It also notifies you if any are missing.
HNP_PreNoteFit_v001: It's complicated to describe what this plugin does, but it basically improves the pre-note functionality.
Humanizer: Helps achieve a human/realistic result when tuning.
hyphen: I haven't been able to find exactly what this does.
resampler_patcher1.4: Makes resamplers run faster, recommended for resamplers like wn4u (ONLY WORKS ON "CLASSIC" RESAMPLERS, WILL NOT PATCH RESAMPLERS LIKE HIFISAMPLER)
iroiro2: The best plugin for classic UTAU. It's basically a Swiss Army knife, with an insane amount of features.
JapaneseLyricConverterForVocalSynths_v120: Converts romaji to hiragana on a wide range of synthesizers.
Kaoemoji: This plugin is useless but cute; it allows you to add notes with kaoemojis.
kbf110731: Improves several note-related features
longrcutter_1.01: Splits long rests, which I find quite useless
