Here are several ways you can use PVC pipe in your garden:
1) Trellis for Tomatoes and Peppers
In a big enough yard, you can use PVC pipe to build really big frames. In this garden I once attended to I used some smaller diameter pipe to act as a stake to allow the longer and wider diameter pipe to stand upright. Once the structure is standing, some string or other method of tying plants up can be used to keep plants upright. Read more about it here.