Run by the same woman who runs Fantasy Name Generators, Roll for Fantasy is the sister site where you can find a plethora of useful tools. You can create maps, dungeons, coat of arms, loots, etc. There are a ton of guides ranging on how to deal with player death to puzzles, and writing guides. Speaking of puzzles, there's a section for those too! And a fun DIY section for crafts you can do at your table.

This website has it all- DND speific name gens, general ones, modern, sci-fi, cultural, and so forth. It also includes name generators for things like fantasy species, aliens, diseases, clans annd more. It has a feature that allows you to save the names you like as well. The site also features a plethora of other generators such as; backstory, apperance, personality, towns, landmarks, prayers, and so many more. The site is constantly updated, and as far as I know, it's also run by one lovely woman!

An extremely useful tool with tons of the monsters from DND. it allows you to build an encounter, and tells you the general difficulty of the encounter. I find myself using this to check and see if something is going to be too easy or too hard. It includes monsters from pretty much any source you can think of, a robust search engine (sort by type, challenge rating, etc), an option to import custom monters and allows you to send encounters directly to Improved Initative.

Another extremey useful site, Improved Initative has everything you could ask of a combat tracker. HP tracking, initative order, AC tracking, status tracking...it goes on. There's two views- a DM view, allowing you to see the full details of your monsters and players, and a player view that shows all PC health and AC, but only displays "healthy", "bloody" (etc) and the name of the monster. The DM can add tags to the monsters, reveal their AC, and so forth, that the players can see. While this tool requires a bit of set up, I have found it useful as a DM and as a player! It easily displays who goes when in combat. Addtionally it comes with the statblock for the DM to reference for monsters.

Maybe you didn't want to carry your books to the session. Maybe there isn't enough room, or you can't flip fast enough. Maybe you don't have access to them. Don't worry- the WikiDot has literally everything you could need. All the 5E (not DNDOne or whatever they're calling it) classes, races, backstories, feats and even unearthed arcana. There is spells too, although I'm uncertain how many. I find myself, and my players using this site for quick reference or for building characters when the books aren't near.

I find myself using 5E Tools moreso as a DM than player. It has nearly the full beastiary for all of 5E's sourcebooks, and the same with spells! it also lists classes, feats, etc, but I find the layout to be more suited for comparison (and I personally have trouble navigating on mobile). I find though, that this is the best site (at least for me) for referencing spells and magical effects!

A simple, sleek spell tracking app. It allows you to create a caster in the app, and automatically does the math for your spell slots, attack mdoifier and so forth. Choose your spells, toggle if they're known or prepared, and track spell slots all in one spot! I also use this as a quick reference tool as a DM, seeing as the app functions basically as a spell library if you aren't using a caster.

Okay, even outside of DND stuff I love Notion. Imagine having an endlessly customizable notepad to keep track of everything, AND it can function as your own Wiki. Don't let their advertising as a buisness mangement tool put you off- plenty of writers and creative types use it too! I essentially built my own Wiki within the app for my DND world to keep track of everything. You can create pages for your world, databases of your characters, notes from your last session, and more. It's too much to get into all of Notion's uses, but I recommend it.

Similiar to Notion, this is a notetkaing app of sorts. Essentially you create bubbles and link them to other bubbles, to show their relations. The UI is tricky at first, but incredible once you understand. You can add more detailed notes that only appear upon clickng a bubble as well. I'm far less experienced with it, but I did dabble and found it quite astounding.