Although they can be problematic at times, floats work pretty well for a simple layout. Here,
main is floated left,
nav is floated right, and
clear: both applied. Floats do add much more flexibility design-wise, but are limited, and, as stated, can be problematic in (more) complicated layouts.
Floats remove the floated element from the document order, so one needs to be careful about what happens after that element, thus the clear on footer. The switches below will toggle the floats.