I began editing my Starfield cart to remove the parts that aren't necessary and fix a couple things. So here's a few cartridges that you can modify and drop into your own programs, as well as a tweet length (exactly 280 characters!) starfield cart.
Edit: moved this to code snippets where it belongs.
Starfield Snippet: this cart is medium size, including more rendering code so you can do things like modify the position of the camera and including the function that allows the user to change the speed of the stars. Also fixes a color bug in the original and adds a new dark blue transition color as suggested by @Liquidream.
Multicolor Starfield Snippet: Starfield Snippet, but with a colorful effect inspired by the 70s warp effect in Star Trek: The Motion Picture.
Mini Starfield Snippet: this cart is smaller, removes unnecessary stuff, and streamlines the rendering, meaning you can't as easily change the position of the camera, the controls are gone, and the color transitions are removed. Instead you can write your own code to handle these functions how you like.
Tweet Starfield: a tweet-length starfield. Took a little bit of time to get it down to this many characters, and I don't think I can make it any smaller, though I'd like to see what others can do. Missing color effects and controls, and the star streaks are only as large as the distance the stars travel, but it's the real deal.
A starfield effect, inspired by the old Windows screensaver, though the style is more in line with Star Trek. Use up and down to increase and decrease speed, and press X to show stats.
I used nucleartide's Pico-8 snippets, specifically vec3 and pline(), to do the 3D projection. I was encouraged by reinvdwoerd's Perspective Lines and used his cartridge to figure out how to use pline().
To really get immersed, shout "increase speed to [warp number]!" as you hold the up key or "all stop!" as you slow down to zero.
An attempt to replicate the Star Gate effect from "2001: A Space Odyssey." Use Z to turn the 'music' on or off.
I was messing around with my earlier fire demo and found this weird result. Use left and right to adjust the width of the bars, up and down to adjust how fast the bars change colors, X to change the colors randomly, and Z to reset. The rendering is interlaced, which leads to interesting results when you hold down X -- epilepsy warning!
I don't see this being particularly useful, but maybe it could work as a music visualizer.
A real-time fire effect based on the PSX Doom fire described by the wonderful Fabien Sanglard. Uses interlaced rendering to speed things up dramatically. Use the arrow keys to add wind, Z to show/hide stats, and X to turn the fire on and off.
A similar Pico-8 fire was done by Ben Hickling. However, that one looks and works a little differently, and runs at a lower 32x32 resolution (though with higher performance).
I think there's probably ways to make this a fair bit faster, but I just wanted to share this with you all. I hope it keeps you warm, and Happy New Year!