Wooly Thoughts are a small online company who design and sell patterns for amazing optical illusion knitwear.
However, the company has also designed knitwear specific illusions that use raised stitches that involve two colours, one of which is only visible when viewed from the side.
This means that when seen from the correct angle, images jump out on what would otherwise seem to be little more than a plain striped pattern. If that wasn’t awesome enough, some of the patterns are for MC Escher designs.
UPDATE: Grabbed from the comments – thanks Alice!
I’m a big fan of illusion knitting (scroll down for v rough video [of a knitted DNA strand illusion!]):
The people behind woollythoughts.com do some of the best I’ve seen. Their “moonrise” is incredible (it can be quite hard to make the illusion knit patterns work).