Here's a list of various resources I've found that I think are helpful!
Bulbapedia
-- This site holds a lot of information about so many different things about Pokemon, including game glitches, fandom stuff, TCG artists, and merch.
Serebii
-- A great place to go to when needing to quickly find information on Pokemon battle information or just want a look at what's been going on recently. It's also pretty mobile friendly.
Official Pokemon Site
-- Just the official Pokemon website.
Pokesymbols
-- A neat page that lists every single Pokemon TCG set symbol for easier identification.
Let's Find Pokemon! FOUND
-- A super neat site dedicated to finding, documenting, and archiving the Let's Find Pokemon series of books!
Blue Moon Falls
-- An amazing site with really cool tools that mainly focuses on Gen 2 Pokemon games! It has some stuff on Gen 1 and Stadium too.
Buried Relic
-- Guides, tools, and resources that mainly focus on older pokemon games like RSE, FRLG, and Gale of Darkness!
Pamtre Berry
-- A site that's working on recreating the Gen 3 and 4 minigames! It also has a couple of guides too.
Hallowed Tower
-- This site has a focus Gen 4 games with it's several guides for them!
Poke Wiki - Secret Code Generator
-- A secret code generator for the Gen 2, 3, and 4 games! The rest of the site is in German without any translations, hence why I'm just linking the generator.
Pokequestrecipes
-- A wonderfuly made site that makes it super easy to find the best recipies to attract whatever Pokemon you're looking for in the game. This is only for the English version of the game.
Ditto Dan
-- A super neat site that focuses on collecting everything there is for the Pokemon, Ditto!
"This isn't the end. I'm going to be the greatest ever, because these guys are behind me." - Silver, Pokemon GSC