My guess would be a Crataegus, commonly called a Hawthorn.
There are many varieties.
Mike

Thanks, I have a Hawthorn tree, but its leaves are so different!

I could be wrong. Wouldn't be the first time. :-)
I was going mostly on the thorns. There aren't many trees with thorns that grow here. Quince and Locust are ruled out. The leaves are waay different.
I don't grow any Hawthorns because of their many problems so I can't go out and look at their leaves.
My guess it's an English Hawthorn, Crataegus laevigata.
'Paul's Scarlet' used to be commonly planted here, along with several white forms. Not so common now.
Some have escaped and can be a problem in some areas.
Where are the experts when I need em'?
Mike

Yeah, I also have a quince, so I was pretty sure it wasn't that. Hm....maybe I'll have to look it up. There are two of them growing in the same area with some other volunteers that the birds have bestowed upon me. :)

Liable to be Crataegus monogyna, a common weed in the region. Where both this and C. laevigata might be present, the hybrid between them, C. x media could be possible as a spontaneous seedling as well. Most double-flowered hawthorn cultivars are now considered to belong to this cross.

Hawthorn authority J. Phipps claims the true, strongly red C. x media 'Paul's Scarlet' is not present in North America.