TypeCheck
If weaknesses and resistances were a 1-to-1 trade off I would have nailed the type effectiveness chart long ago. But because of variances like "A resists B, but B is not weak to A", I never really got the whole idea down. So I made a small web app that allows you to select a pokemon and it explicitly tell you which move types to use and avoid. Never got aroudn to adding Megas to it though...
ChampDex
Pokemon Showdown and Pikalytics (amongst others) are useful in comparing how one Pokemon will compare against another Pokemon. But I found I was looking for something more specific - if Pokemon A is attacking, which Pokemon would be best against it? Now, this could be done on Showdown on a 1-by-1 basis, but I wanted a full list I could explore that told me precisely which Pokemon could tank a specific move from a specific Pokemon. So I made a spreadsheet to explore this.