A search for moon yields the following results: boon, croon, dune, goon, loon, noon, prune, rune, soon, swoon, tune. What kind of online tool is being used?

It is being used a rhyming dictionary. All the words mentioned represent the same sounds, they rhyme among them. This kind of dictionary is generally used by poets or writer in order to improve their writing and find relevant rhymes for their pieces of writing. It is used for poetry and lyrics.