I think it's a perennial geranium.

It is a perennial Geranium.

looks like a hardy geranium-Striatum is the variety I think. How big does it get?

I have a purple,pink and a white pelagonium geranium called (cranesbill). They can get huge if not divided.

Perennial geraniums (Cranesbill) are the basis for my back garden. VERY easy to grow and you'll have plenty to share with your friends.

Brother-in-law has these, they are very hardy and takes abuse well. My dog has more than once got her tie out lead tangled around his plant and they recovered every time. He lives in northern Michigan and the cold winters hasn't bothered them either. I transplanted a small plant that my neighbor gave me her plant when she moved. Just dug up the whole plant with no prep before hand.

Another great thing about perennial geraniums is they bloom seemingly forever.

I have a couple of these in my yard and always wondered what they are as well. Thanks for the info!

It is a wild geranium--they come in several colors.

Is it like a ground cover?

Nancy, it certainly can be in the right place. They don't creep like a vine, but more like a clumping perennial. They are really cool plants coming it all sorts of leaf shapes and flower colors. Most of them stay pretty small no taller than 18" -24", and some are smaller. Most of them also give a lovely reddish fall leaf color. I give you fair warning though, once you have one beautiful specimen you will soon be wanting more!