Besides 3D Printers and Pens, there are many tools that I consider essential for 3D printing and beyond. Here are some of my selections :

• E-6000 industrial adhesive : While many people prefer cyanoacrylate (super glue) for connecting 3D printed parts, I find E-6000 to be an excellent alternative. It dries clear and slightly flexible, so connections are less brittle. A 1 hour working time also makes it less stressful than an instantly curing adhesive. Oh, and it also works on metals, fabrics, and so much more!
• Digital Caliper : This is a tool I use any time I’m designing a model on my computer. I use it to measure parts when building functional parts, and even when making something from scratch, it’s a useful reference to get an idea of the real world scale of my models. For a long time I used a cheap, plastic caliper, but upgrading to a (still rather cheap) steel caliper greatly improved the accuracy of my measurements. The caliper I bought is no longer available, but there are many affordable and highly-rated steel calipers available on Amazon.
• Tweezers : Most printers tend to leak a bit of filament while heating up the nozzle. While you can have the printer draw out a purge line or put a skirt around your model, little bits of filament can still get stuck here and there. A pair of pointed tweezers are great for picking things out from under your nozzle.
• Wire Clippers : Many printers come with a pair of these wire clippers because they are excellent for cutting filament. You want a clean end on your filament to prevent jams, so these are super handy. The cheap ones work fine for most 3DP uses, but if you want top quality go for these KNIPEX cutters.
• Bondhus Balldriver and Hex T-Handles : speaking of premium quality, I got these hex keys at the recommendation of Adam Savage and they have noticeably improved my experience working on my 3D printers and other activities that involve metric screws. The long, weighty form and ball end provides the power, reach, and versatility needed to screw and unscrew like a champ.
• Heat Gun : A small heat gun often comes in handy for quickly melting away strands from a stringy print, or softening a print just enough to make a part fit. You’ll find other uses along the way!
• Cordless Duster/Vacuum : I use this thing on a weekly basis, for cleaning up my printing space, my desktop, my PC case, my car, and on and on. While there are cheaper alternatives, I’ve found this version to be worth the power it provides. Oh, and it’s great for a humane catch-and-release of spiders too… that alone had me buying several of these!