Alright, I'll join the fun in case any of these haven't already been said or linked. I've learned a lot over the years and there's a few that I can't believe I ever lived without. I'll try to rank them by necessity in hindsight, though not necessarily the order in which I discovered or acquired them. Always try to search and see if you have a local hobby store you can support first when searching for supplies, but otherwise, I've included Amazon or direct links to a lot of these. Some may be regional, but will at least give you an idea of what to search for in a marketplace available to you. I'll try to add more if I get bored, but these are all products I have purchased and really gotten a lot of utility out of. We'll start with trimming, but I do love editing posts to improve them.
A Respirator or Dust Mask (at minimum)
So one thing many of the tools below have in common is that they love to make a lot of dust, so I would be remiss if I did not include a link to some PPE. I recommend the highest quality you can afford - remember, you're not just avoiding breathing in the dust that is created when you sand ABS, but also the vapors that are created as it heats up. I wear a half respirator whenever I am sanding or messing with chemicals or ABS. I've had cancer once, I don't need it again, thank you. Protect your lungs, wear a mask.
3M Rugged Comfort Quick Latch Half Facepiece Reusable Respirator 6502QL - $29.99 USD on Prime at time of post
and
3M P100 Respirator Cartridge/Filter 60926, 1 Pair - $35.49 USD on Prime at time of post
A Dremel/Rotary Tool
Even a cheap one is worth its weight in gold if you accompany it with a nice variety of bits to spin up. There is a wide range of rotary tools, with the Dremel brand being among the more popular (though I myself currently use a Milwaukee M12 Rotary), in a variety of speed capabilities, battery-operated and corded options (I like cordless), and prices, but this is an essential tool for any maker. This USB rechargeable cordless starter kit is perfect if you don't have one, but upgrading to one with more juice or in your swappable battery family is worthwhile.
Dremel Lite 7760 N/10 4V Li-Ion Cordless Rotary Tool Variable Speed Multi-Purpose Rotary Tool Kit - $64.00 USD on Prime at time of post
Abrasive Flap Wheel Sanders (80 Grit) - $7.89 USD on Prime for a 10-pack at time of post
A rotary is only as good as what you spin up with it, and these are by far my favorite tool for final trimming armor after rough cutting with lexan shears and/or the belt sander. Because of their size, you can get nice clean lines and curves, and variable cut with the amount of pressure you apply. I've started using these way more than the traditional drum sander as shown above. Just make sure you're wearing a respirator.
Emory Boards / Sanding Sticks
These were a game-changer. So obvious, but I saw Adam Savage using these in a Tested video and I haven't really used sandpaper much since. There might be a better link out there, but these ones hold up well and have two medium grits (180/240) to choose from. I often use them to dress the edges of my TK kits (in addition to the Shaviv tool Caleb linked above) so they're smooth and don't cause armor bite or wear on my undersuit.
ZMOI Professional Nail Files (180/240) - $7.99 USD on Prime for a 12 pack at time of post
A Belt Sander
A belt sander is so helpful for creating smooth, straight lines... so key in TK assembly. Most of your armor pieces join at flat lines or have gentle curves that a basic belt sander can trim handily. I use this thing for a ton of other projects too. When it finally wears out, I'll invest in a nicer one - it's worth it.
Harbor Freight - CENTRAL MACHINERY 4 in. x 36 in. Belt and 6 in. Disc Sander - $84.97 USD online at time of post (always check their website/flyers for in-store coupons!)