Lifehacker makes some recommendations about how to Find Free Shipping Pallets and Reclaimed Wood for your DIY Projects, pointing to an article with 6 Simple Tips To Find Free Pallets and Reclaimed Materials.
We love recycled building materials at TreeHugger and have shown a lot of projects made from pallets. I just think DIYers should be careful where they get these, and what they are made of. Carpenter Nick noted that you should keep away from grocery stores and pallets used for food:
Remember when E. coli was running rampant through the romaine lettuce community last year? The National
Consumers League(NCL) did some testing on shipping pallets... They found that 10% of the pallets tested contained E. coli. Almost 3% contained Listeria, one of the more virulent of food borne pathogens that has a 20-30% mortality rate.
Designer Lori Danelle, who builds with pallets, suggests that you have to watch the wood:
If a pallet is made of wood, there's concern that it may harbor bugs. In order to prevent this, pallets are either treated with chemicals or heat treated.... Both of these methods get the job done and are indicate on the side of the pallet with the IPPC logo. Chemical treated pallets would have "MB" stamped on it, while heat treated pallets are stamped with "HT"
You do not want MB chemically treated pallet material in your home, really. More on TreeHugger: Are Pallets Unpalletable As A Building Material?