As readers of my blog can probably guess, I usually favor scanline and CRT effects on my retro games but there are a lot of folks who prefer sharp-edged pixels. For many of these folks, integer scaling on the Y-axis and 1:1 pixel aspect ratio (PAR) with nearest neighbor (NN) sampling is gospel, but that can lead to weird display aspect ratios (DAR) on many systems, including S/NES, where the PAR is naturally non-square*. This issue is further exacerbated if you want to run your games at 4:3, which is the nominal aspect ratio of CRT displays and is probably most like what people saw playing retro games in their natural habitat.
If you get away from 1:1 PAR/integer with NN sampling, you end up with a lot of problems caused by uncertainty regarding where in the texel (that is, the texture's pixel) the upscaled pixel is actually coming from. This manifests as "shimmering" during what should be smooth scrolling and ugly, uneven pixel sizes on what should be smooth slopes:
Originally written by Fes, this is the OG sharp anti-aliasing shader and has been ported everywhere. It takes four texture samples a small distance from the current pixel and averages them together.
jimbo1qaz took another stab at the same concept and wareya added some bits to allow for interpolating in "pseudo-perceptual" colorspace. It ends up being slightly darker than pixellate via gamma curve, surprisingly enough:
Themaister took a different approach with this one. It prescales the image to a desired integer scale (I added a default option to automatically choose the largest integer that would fit on the screen) using NN scaling and then use bilinear scaling to go the rest of the fractional scale factor. In this shader, there's no averaging of multiple samples, so the gamma status doesn't matter. It ends up landing somewhere between AANN and pixellate via gamma curve:
There are a few other shaders worth mentioning that I didn't include here, like Inigo Quilez' "improved texture filtering" (we just call it 'quilez' in the repos), which is significantly sharper than plain bilinear scaling while still providing evenly sized pixels, and aliaspider's scaling swiss army knife, GTU, which can produce a similar anti-aliasing effect when pushed to a very high internal resolution.
These shaders are available in Cg, GLSL/slang and quark/GLSL formats.
*This statement is hilariously contentious and causes some people to flip their shit.