I got this list from Yahoo:

• Pizza boxes. The oil from pizza can contaminate cardboard boxes, making it impossible to process them into clean paper.
• Napkins and paper towels. It's not the paper goods themselves that present a problem, but the fact that they're typically used to wipe up food, cleaning products, and other "hazardous waste."
• Sticky notes. Their size, color, and the adhesive strip make them a better bet for the trash bin.
• Plastic caps. Curbside programs won't recycle them, but Aveda collects them and turns them into packaging for new products.
• Wet paper. Paper fibers that have been exposed to water are shorter and therefore less valuable to paper mills, making it unprofitable to collect and recycle.

Figuring out which plastics you can recycle is often confusing. It's generally well known that most curbside programs only take plastics labeled #1 and #2 on the bottom, but many people are shocked to hear that shape sometimes plays a role. For example, many communities don't accept tubs (mouth wider than base), but will take bottles (base wider than mouth) even if the numbers are the same because these plastics are manufactured differently, says Darby Hoover of the Natural Resources Defense Council.