1. Stippling (using dots in one color) and pointillism (creating dots in many colors, with a more Impressionistic style from stippling). I'm lumping these two together because they're both equally amazing and similar in style. Recognize that man on the right?
2. The Human Printer: Taking stippling and multiplying the insanity by 100 percent. It's ridiculous! I know this isn't an established "technique", but when I saw the artists' tedious process, I fell in love with it. They slowly dot the piece just like an actual printer would, using only CMYK tones.
3. "The Droste Effect" is one that many of us have known and probably even done before without even knowing that it had a name. It's a picture in a picture in a picture in a... well, you get it. It's a great optical illusion that reminds me of nesting dolls. The effect is named after Droste's packaging-- I don't know about their cocoa, but their chocolate is delicieux!
4. Trompe-l'œil is the love child of realism and optical illusion. The paintings are so real that sometimes, you can't even tell what's real and what's fake! I'm still staring at the painting on the left trying to figure out which parts have been painted.... It's also useful when you want to create space in a small room. Recently I've seen it being used by 3D sidewalk chalk artists, it's ridiculously good!
This post has gone on for far too long, so I'm going to stop here. Click through each picture for the source, and to see them in full size. Which is your favorite? :)