Hepatica acutiloba - Sharp-Lobed Hepatica
Dentaria laciniata - Toothwort 
Claytonia virginica - Spring Beauty
Cardamine douglassii - Spring Cress
Phlox bifida - Cleft Phlox, Sand Phlox
Have you ever looked closely at Glechoma hederacea? I'm not sure that I had, until today. This putrid, creeping weed was introduced for ornamental and medicinal purposes in the 1800s. The flowers certainly are pretty, but I can see it being more sought after as an ornamental if the flowers were two or three times the size that they are.
Cardamine hirsuta is one of those weeds that you see so often early in the year that you begin to not even notice it anymore. It flowers and goes to fruit very early in the year, and in May you can hear the seeds exploding from the siliques as you walk through a field where it is present. This mustard truly does have cute little flowers, even though it is introduced.
Another mustard, Cardamine concatenata is native to mesic upland forests, bottomland woods, and rocky bluffs in the eastern half of the United States. You can often find this ephemeral in large colonies in somewhat degraded situations as well as in more intact systems.
Ahh, Claytonia virginica, the genus of which was named for 18th Century botanist John Clayton, certainly is a beautiful sight in spring. Being one of the earliest blooming native plants in this part of the country, my photo library is filled with pictures of this purslane. With a range similar to the previous species, spring beauty can be found in intact woods as well as in disturbed areas such as mowed lawns. The tubers are edible (but should first be cooked), and though small, taste like potatoes.
If you're walking through the woods in April and not paying close attention to the forest floor, you may walk right past Erigenia bulbosa. The flowers of this member of the parsely family have petals up to only 4mm long. Erigenia litterally means "born in the spring," which is why this plant is commonly called "harbinger of spring." The tuber referenced by the specific epithet is supposedly edible, though I've never tried it.
Caltha palustris, one of the more gaudy of the spring wildflowers, is often found in swamps, seeps, and along creeks. It is difficult to not notice this plant in April as you drive past wet areas, as the golden glow given off by a colony of marsh marigolds grabs your attention. This buttercup is native in the northern half of North America, as well as in Eurasia. (By the way, check out that tiny insect hanging out on the petal-like sepal.)
Behold, the harbinger of spring - Erigenia bulbosa! This photo was taken on March 19, 2005 at Fall Creek Gorge in Attica, Indiana.
Trillium nivale, aptly named snow trillium, as it can sometimes be seen flowering while snow is still on the ground. This photo was taken on March 19, 2005 at Fall Creek Gorge.
The ubiquitous spring beauty, Claytonia virginica. In the depths of winter, even the most common of plants can bring us joy. This photos was taken on April 1, 2006 at Mammoth Cave in Kentucky.
When there isn't much florally happening on the forest floor in early spring, Hepatica acutiloba is at its best. This photo of sharp-lobed hepatica was taken at Dowagiac Woods in southwest Michigan on April 15, 2006.
Upside-down, they do look like a Dutchman's breeches, don't they? I took this photo of Dicentra cucullaria at Bendix Woods in South Bend, Indiana on April 10, 2005.
If you've seen the roots of Dicentra canadensis, you understand why it's commonly referred to as squirrel corn. This photo was taken on April 17, 2005 at Bendix Woods.
In mid-April, marsh marigold lights up forested wetlands and seeps. This photo of Caltha palustris was taken in LaPorte County, Indiana on April 19, 2006.
Bloodroot, Sanguinaria canadensis, is beautiful even prior to anthesis. This photo was taken at Waldhaus in Buchanan, Michigan on March 25, 2007.
And a final sign of spring, the leaves of Asarum canadense, wild ginger, just beginning to unfurl. I took this photo at the Heron Rookery in Porter County, Indiana on April 1, 2007.