It's got to be frustrating to deal with runoff from your neighbor's yard. Developers are supposed to design sites so that water is managed on each lot rather than shifted off to the neighbor... With a large red maple in a small back yard, you won't be able to level the yard without damaging the tree. Ninety percent of a tree's roots are in the top 6-12 inches of the soil, and they extend out from the trunk a distance 1 1/2 to 2/1/2 times the height of the tree. That means you've got tree roots under the surface of your entire back yard. Hauling in soil to level things off will smother the roots, and removing soil will physically damage and kill roots. The best thing you can do for your site is to create a rain garden. It's a garden of deep-rooted, moisture-loving perennials planted in a shallow basin-shaped bed. The idea is that rainwater runs into the slightly sunken bed where it pools and then percolates down into the soil profile. Rain gardens are good environmentally, but they also solve water problems on site rather than just shifting the water off to become a problem somewhere else. (Many large retailers are starting to incorporate rain gardens--also called bioswales--into their large paved parking lots.) Regardless of the size of your yard and your maple tree, a rain garden can be incorporated on site. There are rain garden plants for both sun and shade. I hope these suggestions are helpful. Please submit another question if I've left you with too many loose ends.