ElvUI Plugin Framework: https://www.tukui.org/forum/viewtopic.php?f=9&t=15
ElvUI Layout Installer Framework: https://www.tukui.org/forum/viewtopic.php?f=9&t=1025
Blizzard FrameXML Live: https://www.townlong-yak.com/framexml/live
Blizzard FrameXML PTR: https://www.townlong-yak.com/framexml/ptr
Globe Search: https://www.townlong-yak.com/globe/wut/
⤷ globe is very useful for gathering information about undocumented addon api, widgets, events, etc.
SublimeText_ColorHighlighterLua [by Resike]: https://github.com/Resike/ColorHighlighterLua
⤷ plugin for Sublime used to show color in API and |c tags
SublimeText_WoWDevelopment [by Resike]: https://github.com/Resike/WoWDevelopment
⤷ plugin for Sublime used for lua linter, autocomplete, and syntax highlight improvement
BlizzardInterfaceResources [by Resike]: https://github.com/Resike/BlizzardInterfaceResources/
⤷ some data files like api, events, removed events, cvars, templates
BlizzardInterfaceCode [by tomrus88]: https://github.com/tomrus88/BlizzardInterfaceCode
⤷ Interface folder containing AddOns, FrameXML, GlueXML, LCDXML, and SharedXML
⤷ a majority of api, events, cvars, etc are documented here, as well as some general information.
Battle.net CDN mirror: https://bnet.marlam.in/files.php
⤷ this site was made to archive all CASC builds of a specific set of Blizzard games that have been available on Blizzard's CDN.
Another AddOn/Coding related discord server: https://discord.gg/u68HY5U (check out their #resources)
WoWUIDev Discord: https://discord.gg/7EMfzHw