I put together some comparison snaps of scaling options in the GENS (Sega Genesis / MegaDrive) emulator. These are 2x-pixels scaled up in six different methods: pixel doubled, scanlines (50% brightness), and four different resampling methods. These methods are detailed (often excruciatingly) over on Wikipedia.
For your viewing comparison, I've doubled these images up again, so you can see how the scaling algorithms affect the image. As you can see the resampling methods are quite different: the interpolated system essentially blurs the images, where the SAI and SuperEagle try to knock off corners and create diagonals where none exist.
There are other methods available, and some emulators go to great lengths to create a pleasing image for the player. In lieu of perfection, my favourite method is hard-pixel scaling, 'cause it suggests the most clarity. Sometimes though, you want to go the extra mile. The SNK and Capcom emulator Kawaks will even render the game using a multi-coloured faux-CRT shadow-mask filter that is so extreme you can't help but love it:
Here's another shot, from Capcom's Ghouls n Ghosts, showing CRT emulation, and raw pixel doubling. For comparison, the last shot is my 1/3 scanline method, which I gotta say looks pretty sweet. =D
Finally, here's something I just tried and immediately fell in love with. First, the lesson: on the old CRTs, a brighter signal (more power to the firehose) would result in a wider scanning beam, which in turn would create an illuminated line that was thicker, and would encroach on the dark spaces (scanlines) between the beams.
Here's my attempt to emulate that:
Man, how awesome does that look? =D Speaking of silly modifications to game graphics... I made some crazy tweaks to Yoshi's Island the other day...
Edit: greets to Kotaku, gizmodo, and gonintendo readers, and to everyone else... Especially you!
Edit 2: My wife just told me a story about how, when telling her friends about my pixel art book, they didn't quite get what a pixel was. She explained a pixel to be the square, pointy things that mario is made out of. They almost got it, but couldn't get the idea of a whole book about it. "What does it discuss? How 'this top-right pixel on Mario's hat is extra sharp' or something?"
Edit reason: Added extra edits.