Good call, Scott! It is Viola pedata, Birdfoot Violet, a denizen of the dry country. It's especially at home in dry sand among a thin growth of Black Oak and Sassafras in the northern third of Indiana. Charles Deam also found it on sandstone ridges in a few counties along the southern edge of the state.
Birdfoot Violet at Ober Savanna Nature Preserve, Starke County, Indiana
The winter leaves are markedly different than those of the growing season. In winter, the leaves are coriaceous (thick and leathery), on very short petioles, usually very purple, and the lobes are short and wide. New leaves emerge in spring with long and very narrow lobes. Here's a photo of purple winter leaves and the old, withered leaves lying on the ground.
This is Viola novae-angliae, a species of boreal forests in Wisconsin that is also known from Manitoba, Ontario, New Brunswick, Minnesota, Michigan, New York, Vermont, New Hampshire, and Maine. Gary mentioned that it used to be state-listed in Wisconsin; it is common at this site.
Vicia americana is a plant found in a variety of upland habitats in Wisconsin. It is also known from much of North America.
I saw Maianthemum trifolium (=Smilacina trifolia) for the first time ever on this trip. We first saw it in a roadside bog near Wascott, Wisconsin. At our site, it was growing in Sphagnum spp. within the black spruce-tamarack restoration area. It is known from all provinces in Canada, as well as from New England and a handful of northern states in the United States.
Rubus pubescens is an herbaceous, thornless raspberry species of bogs, conifer swamps, boreal forests, lowland forests, sedge meadows, and pine barrens. It is found in nearly every county in Wisconsin, as well as throughout most of Canada and the northern United States.
This is the beautiful sedge Eriophorum angustifolium. This cotton grass is known from the northern half of North America, as well as from a few southwestern states. It grows in a variety of wet habitats.
You may have seen my recent post on this species, Castilleja coccinea. At this site in Superior, Andrew and I found just a single Indian Paintbrush plant. In Wisconsin, this species is found in boreal forests, lowland forests, prairies, and on sand dunes.
We saw numerous sedges on the site, but one of my favorites has to be Carex castanea. Chestnut Sedge, as it is known, is only found in a few northern counties in Wisconsin. It can be found throughout northeastern North America in cedar swamps, mesic conifer-hardwood forests, conifer forests, and mesic meadows.
Corydalis sempervirens is found throughout the eastern United States and Canada, as well as in Alaska and Montana. It is endangered here in Indiana and is rare in the Chicago Region; this was the first time I've ever seen this species. Rock Harlequin is found in sandy soil following a disturbance (fire, scraping, etc.), and it doesn't persist long in the absence of disturbance. In Wisconsin, it is found on cliffs and in savannas. We found it on the site in a burned area.
Viola lanceolata - Lance-Leaved Violet
I have heard that this violet really is not separate from the regular common violet, even so, it is still fun to see. This was growing on the rocky, open hillside at Nicholson Preserve, Wayne County, IN.
The forest floor was already alive this past weekend. Most of the white specs in the photo above are Enemion biternatum (Rananculaceae), shown below. Ben already discussed the differences between this species and Thalictrum thalictroides in his post here. Another difference between the two is that Enemion biternatum has conspicuous white, papery stipules.
The other contributor to the snowy-spotted landscape is Trillium grandiflorum (Liliaceae), the state wildflower of Ohio. This charismatic species, shown below, had just begun to flower. It should be at its peak next weekend at Bendix Woods. Seeds of T. grandiflorum possess elaiosomes, which are fleshy, fatty structures. Ants are attracted to the elaiosomes. When ants harvest seeds from different colonies of T. grandiflorum, they are helping to increase the genetic diversity of the future colony that will germinate at the location of their nest.
Both Dicentra canadensis (below) and D. cucullaria (below that) (Fumariaceae) were in flower, though I saw many fewer flowering individuals of D. canadensis. I noticed a lot of bumblebees in the vicinity of these two species, and as it turns out, bumblebees are thought to be the only pollinator of the two species.
Like Trillium grandiflorum, the seeds of Dicentra canadensis and D. cucullaria possess elaiosomes and are spread by ants.
One of my favorite spring wildflowers has to be Floerkea proserpinacoides (Limnanthaceae), pictured below. Why? Probably because most people don't even notice it, and because the flowers, although just a few millimeters across, are rather interesting. Small bees and flower flies are known to pollinate false mermaid, as it is known, and both insects were in abundance at Bendix Woods yesterday.
The maroon flowers of Asarum canadense (Aristolochiaceae), shown below, are low to the ground, where they are pollinated by ants. This has to be one of my favorite ephemerals. Oh... wait... I just said that about the previous species! It really is difficult to pick favorites, but who doesn't love a plant that has roots that can be used to make candy? I made candy from the roots of this and true ginger (Zingiber officinale) in college. While some pieces were too strong, others tasted very good.
Several other species were in flower, but I'll save those for the people who come on my hike next week! 
Violets are a fun and diverse group. Yesterday alone, I saw Viola blanda, V. canadensis, V. conspersa, V. odorata, V. pubescens, V. rostrata, and V. sororia in flower.