Note that while not all these may fit a strict definition of spring ephemerals as they don't die back over the summer, all do come up early in the spring to take advantage of high light levels.
To start off with, Blue Cohosh is an interesting plant named for its distinctive blue berries later in the summer. We actually have two species: Caulophyllum thalictroides has yellowish flowers that open later than the purple-flowered C. giganteum.
Wild Ginger (Asarum canadense) has a bizarre flower that lays flat on the ground. I've found conflicting information on what pollinates these flowers (possibly ants or carrion-loving flies), but the seeds are definitely spread by ants, and the flower position may make this easier.
Members of the arum family (Araceae) have very strange flowers, and a few species are found in Ontario. Possibly the very first plant to flower in spring is Skunk Cabbage (Symplocarpus foetidus) (this flower from a week ago is already dying pretty quickly). The flowers actually produce heat to melt through snow and ice the spring. Later on the huge leaves cover the wet habitats in which this species grows.
The other common woodland arum is Jack-in-the-pulpit (Arisaema triphyllum).
Two similar species of Dicentra can be found blooming together: Dutchman's Breeches (D. cucullaria) and Squirrel-corn (D. canadensis).
Dutchman's Breeches flowers look like little pairs of pants.
Squirrel-corn is similar, but easily distinguished by the rounded lobes.
There are lots of other ephemerals blooming right now but in the interest of keeping this post to a manageable length you'll have to wait a bit for pictures of them.