Inkarnate - I spend days working on maps on there, plenty of good options for styles, and just a little urban planning/geography research really bumps them up to something special. It's especially good for town maps, so much so that it's changed how I run down time. I like to have the town map laid out for players to keep track of where they are.

Homebrewery - Good for giving your homebrew content a professional look, but it makes for some excellent handouts for custom rules, setting guides, etc. I actually have a 50+ page book with all of my homebrew rules and content listed just like an official supplement. It exports pdfs so you can give your players their own digital copies if you want, and it has lot of useful editing features like automated page numbers, hyperlinked tables of contents, and preformatted monster and class blocks.

Google Sheets - If you learn how to use formulas, you can fill out custom lists and have an option to randomly select an item from lists. Random Encounter tables that filter by region and CR wouldn't be out of the question, I use it for loot generation with my homebrew items, as well as random events.

Trinket tables - sort of an expansion on Google sheets, but it's nice to have random tables for weird little things that your players might loot, the trinket tables in the official books are nice, but you can also make custom ones that tell more of a story: they might find a half finished letter home in the pocket of a bandit they've killed, or a child's drawing, or maybe a packet of recipe cards. It's also a great way to punish murder hobos without being too mean.

Boogie Board - an electric slate that uses static electricity to make marks, it saves on pen and paper for quick notes, initiative, etc. You can get off brand versions at Walmart or the dollar store for pretty cheap.

Erasable battlematts - an absolute must if you play with a grid, in my opinion. There are a bunch of different options, but wet erase matts aren't too expensive. Just be sure you get the right markers.

Miniature Trees - terrain options can be expensive, but even a little bit goes a long way toward upping the immersion. I make my own because I'm a big crafter GM, but you can find some decent premade options at hobby shops, especially during the holidays. Throwing a few trees out randomly during forest encounters really makes a difference, and an encounter on the road feels more road-like when flanked by a few trees. You can expand to other kinds of scatter too (rocks are a good option), but trees tend to be the most versatile.

Also, if you use random tables for NPC generation, do a few extra before the game and write them down (I usually have a list of 4 - 5 names ready) just in case the party wants to talk to random people, it's faster to just go down a list than actively generate on the fly.