Free Verse
Whitman loved himself some free verse; some might even call him the father of free verse. He was so over regular rhyme and meter. For Whitman, free verse meant Freedom (with a capital "F").
But that doesn't mean that Whitman's poems are a big ol' mess of lines. "I Hear America Singing" is actually pretty tightly controlled. Most of the poem takes the form of a list, and the same phrasing ("The + [profession] + singing") dominates the beginning of many of its lines. Check it out:
The carpenter singing his as he measures his plank or beam,
The mason singing his as he makes ready for work, or leaves off work,
The boatman singing what belongs to him in his boat, the deck-hand singing on the steamboat deck. (3-5)
While Whitman's lines tend to get longer as his poems go along, they are often tightly controlled at their beginnings. We call this type of repetition at the beginning of lines anaphora. This kind of patterning is pretty much the opposite of traditional verse forms, in which lines are often tightly controlled at their endings so that they can rhyme with each other.
In Whitman's hands, so-called "free verse" is never about being all willy nilly. For this poet, free verse was all about having the freedom to discover and create his own forms, which—like a true American original—is exactly what ol' Walt did.