Recycling containers is a great way to save money as well as have plenty of room for all of your plants. But all plastic containers are not safe to reuse! In the US, all plastics are labeled by number.
Plastics labeled 1, 2, 4, & 5 can be reused for planting. Any plastic containers labeled 3, 6, & 7 should be avoided. This seems simple enough, but another thing to keep in mind is what was originally stored in the plastic container? If there is any chance it was used to store chemicals or pesticides or anything else unsavory, it is best avoided, even if it has a safe number. This is one reason to be wary of simply salvaging plastic containers from rubbish piles or yard sales - it's impossible to know what it was used for.
Want more information on plastic containers? Here are some great links that have what each plastic is made of, why certain plastics are not safe, and things to consider when using plastic for food. Check them out below:
EMR Labs: Detailed information on each type of container, by recycling number.
The Micro Gardener: Choosing safe containers for growing food.
Green Roof Growers: Plants in Plastic
Healthy Child, Healthy World: Know your plastics.