Commensalism is the relationship between two organisms wherein one is getting benefits, while the other is not affected, harmed, nor benefited.
Tree Frogs and Trees
The tree frog uses the tree to shade and protect itself from the rain.
Orchids and Trees
They grow on tree (Some), but do not harm the tree.
Spider and Tree
The spider will make its web on a tree, wherein the tree isn't benefited nor harmed.
Crab and Seashell
A crab is living on an empty seashell on which the seashell isn't benefited nor harmed.