Natural wood is a very common background material. It is so versatile that it can be used in any sort of tank. Try to avoid grape wood, as it will tend to rot faster than other varieties of wood. Mopani, Manzanita, Malaysian, Cypress & Ghost wood are hard & durable that can withstand the moisture in the tank and last for years to come.
This fire-bellied toad tank uses wood as ledges and a climb out area for the water:
This paludarium also uses wood as a climb out area, but also to grow moss on the water bank.
This tank uses a stump to look like a tree and as a centerpiece to the tank:
I love the way this tank uses pieces of driftwood to look almost like a tree. Moss & air plants would love all those crevasses to grow into.
Here is a tank with no background at all- only a piece of wood attached to the glass and a piece of wood emerging from the ground.
Wood can also be used to create some extra space for the frogs & add some dimension to the ground like in this tank:
What kind of woods do you like to use in your tanks?