If you're not sure what I'm talking about, I suggest reading this Stereogram FAQ.
If you know what they are but are having trouble seeing them, try some of these suggestions.
Stereo Flicks. Some pictures and a GOOD explanation of the theory behind it.
A Stereogram FAQ. HUGE, with A LOT of good information!
Some experiments with multiple depths in one image:
Experiment 1.
Experiment 2.
Experiment 3.
Linez. A neat effect, NOT using patterns.
Another for my brother. A slight variation on the one above.
Circle. A "Random-Dot ASCII Stereogram"
Trees. Using ASCII pictures instead of a pattern.
3-D Landscape. Combines a pattern with 3-D ASCII pictures.