awakened POE trade, lets you ctrl+D an item and see what it lists for on the trade site
Chaos Recipe Enhancer, if you do the chaos recipe early, this lets you highlight and filter what you still need on the fly instead of going through filterbladez, as well as highlighting the items when you're ready to turn in.
Path of Building, importing your character doesn't automatically tick every box you need it to though, sometimes you still need to go through the configuration and check power/frenzy charges, or other things that affect your specific character, but you can import a piece of gear to see whether it would be an actual upgrade or not.
Not apps, but some helpful bookmarks I use:
poe.ninja - pricechecker
Exile Leveling - step by step directions for the campaign so I always know what the next step is and I'm not sitting around reading things or talking to NPC's. You can import a build and it will tell you what gems to buy as well.
PoE Planner - to plan out your Atlas tree
PoeMap.Trade - trade maps you have for maps you need for completion and bonus objectives. with kirac scouting reports and horizon orbs this isn't super necessary.
Syndicate Custom Cheatsheet | El rincón del Exiliado de Ayeleth - alt tab to this when you're deciding what you should do with betrayal members. This whole mechanic needs to be removed from the game, but until it is, you can also import this as a graphic into awakened poe trade so you can press a hotkey ingame to see it.
https://i.redd.it/5mkby3269no51.png - Heist members and their caps/what rewards their contracts give
Economy - poeHow - I don't use this much, but if you really want to play the market it tells you what the profit margins on certain things are. updated from poe.ninja
PoExchange - Service Listings - to buy boss carries, also click on Bulk Listings to buy things in bulk that the trade site doesn't support. This is far easier to use than TFT