I actually use hardware cloth for the sides of some of my raised beds. It provides excellent aeration to the soil while still containing it. I thought it might rust out over time but so far it has held up well for 6 years.
I like to use my raised beds as compost bins when they are not growing anything, and the 2 foot high screen mesh sides allow me to dump a load of leaves, grass, etc. on the beds. Besides, hardware cloth is cheaper than any other non-toxic building material I could find for building my raised beds.