Pressure treated Wood:
There are studies out there showing that the treated lumber does not leach enough of anything into the soil to be a problem,
especially if you are not planting anything right up against the wood.
Also be aware, pressure treated wood today is not the same as it used to be.
The EPA got rid of chromated copper arsenate (CCA) treated lumber back in 2004 for home use.
This is the "bad stuff" because it has arsenic in it.
Treated wood at your local box store is now amine copper quat (ACQ) and copper azole (CA), there should be little labels on the boards, check them.
Either of these should be OK to use for raised bed gardens.
Cinder Blocks:
My sister in law uses these and they seem to work pretty well.
Plastic Barrels:
My wife has gone the plastic barrel route, she likes stack them in our greenhouse when she is not using them,
and they are easy to move and set up in different areas each year till she finds a spot she really likes.
Recycled material:
The box stores are also selling kits made from recycled materials which will not rot and are easy to put together.
Example
Lots of options out there.