I will add to this answer if I think of a resource that hasn't already been added.
Glitch City Wiki is a high-quality resource for glitches in Pokemon, especially the first four generations. Its articles are usually clearer and better-structured than their equivalent on Bulbapedia. The wiki is continuation of Glitch City Laboratories, which was archived in 2020.
ChickasaurusGL is a long-time glitch researcher with an active YouTube channel. If you need live gameplay of glitches, especially for Gen 1, then you are likely to find it there. Other researchers on YouTube I've taken note of are Crystal_ and RETIRE (and there are many others out there). Overall, if you're dealing with glitch questions, YouTube is a good resource, with a lot of documentation that's hard to find elsewhere.
PokeAPI is the most complete source of Pokemon data in formats that are easy to parse. And obviously, it lets you query an API for Pokemon data. You can use their GitHub repo to directly access data for most aspects of Pokemon up to US/UM, after which it gets a bit patchy. You can use it to answer questions like this where you think "gee I wish you could query PokemonDB/Serebii".
Pokemon Reverse Engineering Team (PRET) is a disassembly and decompilation project for various Pokemon games. The code in their repositories can be built back into a working ROM for the target game. Their most complete project is Pokemon Red (and their projects for Game Boy games are very faithful to the actual source code, as assembly code is quite close to machine code).
Project Pokemon is a community mostly focused on save editing, RAM manipulation and romhacking. A great deal of research on game mechanics comes from these areas -- in particular, it hosts PKHeX which is the culmination of years of research on Pokemon save file structures. Lurking their forums and tutorials area and trying hacking software can help you learn how the game works.
(It's worth adding that Smogon is bar none the best source for original Pokemon research, especially research from testing the game directly. But that research is strewn across their massive forum and also moves around often, so it's hard to link one place to start finding info.)
@Sibuna_Switch/Anubis/Lusamine does a lot of research on game mechanics for modern games, and publishes the results on Twitter (index). Other notable researchers such as Kaphotics also use Twitter (and here's his repository).
Bulbapedia walkthroughs (example) are useful to understand precisely what a (completionist) playthrough of a certain game entails. You can use the info to inform advice on what Pokemon are best for in-game playthroughs, and refresh your memory of when certain events happen or when items become available.
Reddit is rarely a primary source of information, but it can useful to read the experiences of other people in certain contexts. You can filter to Reddit using site:reddit.com on any search engine. It is a good idea to find the source of information shared on Reddit, or try things yourself before repeating them here. Reddit is also a good place to direct people asking for things we don't do on PokeBase, e.g. trade requests.
Bulbapedia talk pages (example) are useful to get context around the contents of a page you're reading, such as the technique used to prove a certain fact, and other discussion between experts that could be useful for technical PokeBase questions. The edit history may provide further context.
Smogon's Contributions & Corrections forum has WIP analyses for its strategy dex. These can be useful for answering questions about competitive play. Also useful is the surrounding discussion that can give insight into why information was or wasn't included. Obviously, these should not always be taken at their word as they have not yet passed Smogon's QC measures, and could also be outdated.
Hidden Power of Masuda is Junichi Masuda's (former Pokemon director) blog. It hasn't been updated since 2015, but it has insider info about the development of Pokemon games from Gen 3 to Gen 6. Even better, I actually used it to answer a question once.
im a blisy's YouTube channel and website has demonstrations of RNG abuse in Gen 3, 4 and 5. There is content here appropriate for people with almost any level of experience with RNG. sumwun linked Smogon's written guides above, which are good but also a bit outdated (PokeFinder is replacing RNG Reporter) and a bit verbose.
The Cutting Room Floor has well organised and up-to-date information about cut content in Pokemon games.
The Internet Archive and LibGen store many paid publications, leaks, and other such materials. Parts of this have questionable legality, so I will say no more.
