I alway heard them called "pudding stones"-- from a New Englanders complaint that the stones in his field where a large and thickly studded in his field as raisins in his good wifes pudding..-- i have them too-- there are not to much of a problem because i don't have fields to plow- but i find them all the time when i want to plant something in the garden.

though i am pretty sure the term for them is glacial erratics..