Any good bird identification book will tell you what a particular bird eats. As for websites, I like Cornell Lab of Ornithology (link below). Figure out which birds you'd most like to attract that are in your area at certain times of year. Finches will be attracted to your nyjer seed sock. In the winter, you may get Pine Siskins attracted to that same sock. In an irruption year, you may even get a flock of Common Redpolls in the winter. In winter I like to feed a good homemade suet. It attracts a lot of birds, but mostly I get to see woodpeckers and Bluebirds. I also keep a feeder with Black Oil Sunflower seed (summer and winter) which mostly attracts Tuffed Titmice, White Breasted Nuthatches, House Finches, Rose-Breasted Grosbeaks, and Black-Capped Chickadees. Another thing that will really bring them in during the winter months is a heated bird bath. I never feed the mixes. Most contain corn and millet which attract House Sparrows (a non-native species that I try to discourage). Here is a picture of an Eastern Bluebird, and Downy Woodpecker and a Hairy Woodpecker on a suet feeder in winter.
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Here is a link that might be useful: All About Birds - Cornell Lab of Ornithology

gardening is a big help with many birds as many eat bugs and worms and insects and the garden and flowers will attract them. in spring the orioles and tanagers and grosbeaks eat fruit and jelly and drink necter so an oriole feeder and a necter feeder will bring them in. the chickadees and titmouse and nuthatches eat the suet and many seeds and love sunflower seeds. blue birds and robins both love worms even the mealworms. the woodpeckers love suet and the seed blocks with molasses or syrup in them.
hanging a bowl of mealworms from a hook will bring in a variety of birds. many people don't want to feed the mixes with grains such as cracked corn or wheat or milo as the trash birds like the grains but doves and blackbirds and grosbeaks and buntings also love grains. A bowl or dish of grape jelly or strawbery jam hung from your shepards hook will bring in the songbirds and orange slices or cherrys or fruit bits will also bring in song birds.

Zaphod, I too am in SE Wisconsin. Whereabouts are you? As Kendra said, Cornell Lab's site is a good one. I live in an urban area, and I've been successful at attracting cardinals, dees, woodpeckers, nuthatches, ect. Last spring I was particularly excited when I saw 2 grosbeaks at my safflower feeder. I've found that house sparrows around here will eat whatever I put out, but I too stay away from mixes that have milo or corn.

Oh, and Kendra? That pic is awesome!

I topped off the feeders and added a new suet block yesterday and early this morning I was descended upon by a large flock of Grackles. It was a bit unnerving when I opened my curtains. Little too Hitchcock for my tastes. :)

GoldfinchGuy- I'm in Milwaukee County.

Same here. West Allis to be exact. Grackles tend to not like safflower seed, however it is enjoyed by cardinals, chickadees, mourning doves, nuthatches, house finches and grosbeaks. That is if you want to deter grackles.

I have two of these bird feeders. Is this too small for some of the larger birds? I saw a Red-bellied Woodpecker this morning and it looked to be having difficulty getting at the food.

Here is a link that might be useful: Globe feeder

Did you read the description under the feeder? The first line says: "Outer cage excludes squirrels and large birds, creating a safe place for small birds to feed".

Where's the cut off between large and small bird. Based on the recent evidence Red-bellied Woodpeckers fall in the large bird category. Which birds fall in the larger size category that I can expect to attract? Any recommendations for a good feeder that will work for these kinds of birds?

Plant some shrubs native to Se Wi that produce berries and seeds in your backyard. Also those shrubs will produce bugs that birds will gobble up. A book that I read about birding is Bringing Nature Home by Douglas Tallamy. Zaphod42 A good feeder to use for larger birds is a suet type feeder or a bigger feeder in general.