Erigeron philadelphicus, Philadelphia Fleabane (Common Fleabane)
The upper leaves have rounded bases that clasp the stem.  Up to 3' tall.
This is the only one shown here with flowers that aren't white, but books tell me any of these species could be pale blue or pink.

Erigeron pulchellus var. pulchellus, Robin's Fleabane (Robin's Plantain)
This also has stem leaves with rounded bases, but they don't clasp the stem.  It is more densely hairy and spreads by runners.  This is found only in the Piedmont.  Under 18".

Erigeron annuus, White-top Fleabane
The upper leaves have a tapered base that does not clasp the stem, and the stems are covered with long, spreading hairs.  Up to 4'.

Erigeron strigosus var. strigosus, Daisy Fleabane
A smaller plant with a few narrow leaves, and hairs that lie close to the stem.  Up to 3'.

The accepted name for Erigeron canadensis, horseweed, is now Conyza canadensis.

Sources: The Flora of Delaware, Herbaceous Plants of Maryland.