No website is coming to mind but Norway Maple tends to have big leaves and a white (Elmer's Glue-like) sap in the leaf stems.

Not sure if this will help, but see what you think. It at least gives you the scientific names of many of them, especially the native ones. Once it leafs out, you can always scan a leaf and post it on the Name that Plant forum.

Here is a link that might be useful: Maples at USDA

Thanks for the suggestions. I guess this tree will be easier to identify in a few weeks when the sap is really flowing and the leaves are out.

Does Norway Maple come in a variety with dark purple-ish leaves?

Yes, that is the Norway maple cultivar 'Crimson King'.

Ugh. Well, at least it's not a big tree yet. And if we take it out, that'll mean room for the fruit trees we want. . . Thanks Esh.

Another great tree ID site is this :

Have fun!

Here is a link that might be useful: a dichotomous tree key

Thanks Greenhaven! Woot!