Pokédex Tracker
Hands down the best website to track your progress across various games, with links to Serebii and Bulbapedia for looking up locations.

Pokémon Checklists
I love to 100% games, but Pokémon has never made it very straightforward and this website has all the game and post-game achievements you might want to keep track of.

PokéJisho
Instantly translates the names of moves, items and Pokémon from and to Japanese.

Game Gengo ゲーム言語
Find out how it feels to play Switch games in Japanese by watching the first few hours of gameplay.

Athis' Ribbon Handbook
By far my favourite guide to follow for a step-by-step breakdown of the Ribbon Quest.

Ribbons.Guide
To track those Ribbons on every one of your Pokemon!

Pokémon Ribbons on Reddit & Discord
Invaluable community help available at any time and an excellent archive of past experiences, teams, spreads, etc.

KaeruWFC
How to connect to the Internet using your DS or DSi, allowing you to access the GTS in Gen IV & V.

Poké Classic Network
The unofficial server for anything GTS related.

Smogon Forums' Team Repositories:

• Gen III Battle Tower: Guide and Teams

• Gen IV Battle Tower: Guide and Teams

• Gen VI Battle Maison: Guide and Teams

• Gen VII Battle Tree: Guide and Teams

Pokémon Asset Archive
Extensive, free-to-access archive of Pokémon-related assets, images and materials, maintained by user Lewtwo.