I've been seeing a few questions lately asking people what software they should use to make sprites, and whilst it's possible to make sprites even with just MS paint, specialised software can make it a lot easier to add effects and animate your sprite.
Here is a list (including links) that I've compiled that gives a brief description and it's price. Please note that I am by no means an expert on these software and I'm only giving rough descriptions.
MS Paint (FREE) - The free paint program bundled with your installation of windows. It allows for the resizing of the canvas and can save directly as a png file. Can be hard to animate with and lacks a few features like mirror mode and layers, but generally speaking is more than enough to get you started.
Paint.Net (FREE) - Completely free and powerful software with a large amount of plugins. Very easy to make special effects with if you find the right plugin and has layers, which is a huge upgrade from MS paint. Has a learning curve and it'll take you a while to get completely familiar with the software; as far as I know however, there's no real animation plugin. Used by Lafiel for the ABH mod
Gimp (FREE) - Very powerful image editor and is capable of much more than just pixel art. Like paint.net, GIMP comes with a suite of special effects for you to apply to your sprites and there are a ton of customisation options available. Unfortunately there's no dedicated animation tool so you'll have to mess around with layers to get the animation you want. Also like paint.net, there's a big learning curve before you are proficient with the software. Used by Bubbet
Photoshop ($30USD/mo) - Probably the most powerful image editor on the list, has a lot more applications beyond just pixel art and spritework. One of the few editors that actually has a animation tool with timelines, in addition to the huge amount customisation and effects for you to play around with. Also one of the hardest learning curves. Used by both Meranera and Walt for their excellent sprites.
Aseprite ($15 USD) - My personal favourite, while it lacks the customisation options and the sheer amount of effects other editors provide, Aseprite has the most powerful and easy to use animation editor I've ever seen. You can edit layer by layer, frame by frame and it can chain animations together, all the while looking at a live preview. There are some in built effects which allows for easy lighting effects, but shading is generally done manually. Costs $15 on the steam store, but you can also get older versions for free you search online or compile the source code yourself, it's also relatively easy to pick up and use. Used by me, Bubbet and Ryuuro.
Graphics Gale (FREE) - Like Aseprite, Graphics Gale allows you to preview animations and features some nifty features not found anywhere else like a picture in picture for easy editing of fine details. It also includes an easy way to import art from scanned images, though it's value for pixel art might be somewhat limited. Quite easy to pick up, though it's still slightly harder than Aseprite.
Piskel (FREE) - My suggestion if you've never used a image editor extensively before, Piskel feels like a more intuitive version of Aseprite, but with less options and effects. That being said, Piskel comes with all the important features like layers, animation preview and canvas resizing and is more than enough for the purposes of Cosmoteer for most weapons and animations. It's by far the easiest to pick up due to it's simplicity and can even run in browser if you don't want to download anything.