I know a few of Robert Frost's poems, though I'm not an expert, and I've noticed that they all have long and elaborate rhyme schemes. Is this true for all of his poems? Has he ever written a poem that didn't rhyme consistently (have a rhyme scheme)?
Yes, there are quite a few, including
I'd sooner write free verse as play tennis with the net down.
A review by William O'Donnell says that Frost is "unequalled" by any British or American poets in the 19th century in his use of blank verse. In the early 20th century, such a structure was rejected by Ezra Pound and T. S. Eliot, although Frost continued writing the same way.
Iambic pentameter, of course, permeates Frost's work. However, it is an odd, even informal variant. A contemporary analysis of Frost notes that "The Runaway", while using the meter, is irregular even in the first few lines:
Once when the snow of the year was beginning to fall,
We stopped by a mountain pasture to say, “Whose colt?”
The first line feels slightly smoother; the first, fourth, seventh, tenth and thirteenth syllables are all stressed. However, the second does not flow quite so well.