The free verse
Instead of using metre and arranging lines into rhymes and stanzas, a poem can be written in free verse, that is to say without a recognizable metre or regular rhyme scheme. The lack of formal elements of traditional poetry is compensated by other devices, like repetition, alliteration, assonance, parallelism and unusual line arrangement.
Of course the choice of the free verse is often strictly related to the meaning the poet wants to give and to the aim he wants to achieve.
From Whitman on, many modern poets decided to break away from the traditional conventions of rhyming patterns and adopted the free verse, both for a more aesthetical need for new experimentations, and for giving a new feature to poetry, also from a social and political point of view.