There are two kinds of tools for DMs/GMs and RPGs.
I started old school, so hand written notes and notebooks. They have the advantage of working without power, but are fragile where water and fire are concerned. I have the combat wheel from Dragon Magazine. I photocopied it and pasted it to some cardboard. DM screens, dice bags, book bags, milk crates, boxes, graph paper, hex paper (I still have several sheets of the hex paper TSR put out in the 80s.) battlemats, miniatures, pens, pencils, markers, dry/wet erase markers.
Electronic tools for note keeping, PDFs of rules, graphics programs, mapping tools, random generators, websites of others to glean ideas get maps, CampaignWiki, spreadsheet programs, word processors, genealogy programs, etc. Web-based tools for campaigns, social media for online games, rpg table software – game table?
I wrote my own dice roller on a TI-99/4A in BASIC. I could specify the number of dice and how many sides. I rolled up 1,000 kobolds, 1,000 orcs, etc.
When I got back into building my campaign, I also used NoteTab to build outlines and clips to help stat NPCs, Kingdoms, cities, etc.
Electronic tools have the advantage of carrying more in a smaller package and if well organized, easier to search. downside – Requires power/long battery life, not easy to let a friend look at your PH while you are consulting the map, or DMG, etc. Not good for playing around a campfire.
I have a Galaxy Tab tablet and I like it for PDFs of my RPG books. I can also convert my notes and other information to PDF and use it. This is really useful for the RPG books I can easily get in PDF, but have not found locally, or turned to eBay. I did buy a second 1e Player’s Handbook on eBay, so my original will last longer.
I have used a GTD Wiki, Campaign Wiki to put together some information on my campaign. I have yet to move it to my Galaxy Tab and see if it will meet my needs.
I have found a dice roller that lets me specify the types of dice. It will be useful to use to roll up stuff over my lunch break at work, so I don’t need to haul along dice and all my manuals and notebooks to work on dungeons, etc.
I recently found three helpful apps, OS RPG Tables, OS Monsters, and OS Spells from Appbrewers. The RPG tables are mostly player related tables and combat tables. Monsters are from all the books and some OGL creatures, and spells are all the 1e spells and cantrips and can generate random spellbooks or you can create your own spell book. I will do a more in-depth review of these in one or more posts.
There are lots of tables for different situations on blogs, some just on a page/blog article, or others in free PDFs. Some bloggers even have free compilations of all their stuff. One thing I have found, bloggers that might have good ideas, but no search box and no tags, make it hard to find all of their good stuff.
Then there is RPGNow and DriveThruRPG with tons of free and low cost PDFs. I will discuss some of the things I have acquired over the years, in addition to 1st Edition AD&D Manuals.