What are some good ones to use? Particularly free ones.
Robust, easy to use/understand, feature-rich, etc.
I wanna try my hand at it.

I always hear good recommendations for Krita whenever people want something free.

From my experience-based opinion

On Desktop:
Krita: Free, constantly updated, and you can adjust the shortcuts to make it like any art program in the settings.
Sketchbook: Get the version prior to its split with Autodesk as the newer ones is paid and doesn't have the Copic selections.

On Android:
Sketchbook: The newer versions are at least free to use, with the color selection updated with deeper colors.
Medibang: Also a good app if you want the look and feel of Photoshop, Clip Studio, or SAI without burning tons of cash.
Ibis Paint: Had a hard time using it; must be from my tablet's part having difficulty running it, but I heard it's a good app for illustration.

Old classics that people will probably gripe about:

• The GIMP – Free all-purpose graphics program.
• Blender – Free CG modeling and game asset-making program.
• Inkscape – Free vector graphics program.

I only know about gimp and microsoft paint.

There are crazy draws you can do with microsoft paint, but with that being said it is archaic.

FireAlpaca's another option, but I myself (I don't make furry content so there's nothing to show off) use Photoshop and Aseprite ("Libresprite" is available as a free alternative.)

clawstripe said:

• The GIMP – Free all-purpose graphics program.
• Blender – Free CG modeling and game asset-making program.

*Inkscape – Free vector graphics program.

having used Gimp for 8 years now, i think its excellent but not for drawing. its great for image manipulation and editing.
for vector graphics and graphic design i recommend Serif's Affinity designer. it costed like 30€ when i got it.