You have poison ivy.

Never seen poison ivy with leaves showing that kind of indentation. Could it be Boxelder seedlings?

Maybe, possibly. Put on some vinyl or nitrile gloves and pick some leaves and put them into a sealable plastice bag (Ziploc is one brand name) and take them to your counties office of your state universties USDA Cooperative Extension Sevice for id. Be sure the leaves do not contact any part of the exterior or the plastic bag and turn the gloves you used inside out, without touching any part of the exterior of the gloves, and put them into your trash.

Here is a link that might be useful: Poison Ivy

The picture on the right contains a tree seedling and the bottom one contains a polk plant.

Kimmsr's suggestion if good if you can not find similar pictures on the net for any plant you do not recognixze.

Thanks, everyone. I'm mostly reassured but I will bring a sample to our county extension service.

The picture at top left looks like PI. I've seen a lot of variation in PI leaves and growth habits so I would not rule it out.

Looks like PI to me too. The tree seedling in the second pic looks like elm.