Hey everyone!
I've been playing around with some random tools, some of which I've written myself and others that I've come across. Some purchased and others not. But anyway, I wanted to share some of them in case others find them useful!
Pixel Fx Designer (https://codemanu.itch.io/particle-fx-designer)
This is a great tool that lets you easily create particle effect driven spritesheets. In about 1 minute I was able to define a bunch of particle effects to make a nice flame...
Then export it to a spritesheet for easy importing in a game.
You can try for these kind of effects if your game engine has a particle engine built in, but I always found messing with them time consuming and difficult to get the look that I wanted. This tool lets you see the changes as you make adjustments and I like pulling the effects in as spreadsheets.
CodeManu (https://codemanu.itch.io/) has a bunch of neat tools available for sale but I find Pixel FX Designer the most useful (although Juice FX is also a lot of fun)
LDtk - Level Design Toolkit (https://ldtk.io/)
@deepnight made this great level editor (available as a pay what you want model https://ldtk.io/). I have been really happy with the autotiling rules that you are able to create. Whenever I make a 2d platformer I always create a simple collision layer and then add the graphical layers on top of that and making the changes to the level means a lot of individual tile work. This takes care of all that for me and lets me see how the level is going to look inside the editor.

Depending on your engine/framework (if you use one) you might have to write your own importer. It can output as a Tiled .tmx file in a pinch, but I'm using Phaser so working with the native json file has been pretty easy. I'm about 75% of the way through an importer for Phaser and will be done before the start of the jam tomorrow.
Image Outliner (I wrote this)
I wrote a simple tool that just looks through a folder and outlines all the images that it finds with a solid color. I found that I was spending a lot of time doing this manually, so I took half an hour to write a program that does it in batch. Anything I can do to write these kind of tools before the jam is just going to save me headaches later on.

Any other fun tools people have been messing around with?