It's a maple in flower. It could be Acer platanoides, but the extra little appendages on the leaf lobes suggest to me that it's Acer cappadocicum ( http://www.botanicalstockphotos.com/img17302.htm ).
Norway maple, in both instances. Coliseum maple produces 5-lobed leaves with no marginal teeth. http://www.bing.com/images/search?q=cappadocicum&FORM=HDRSC2
Ditto to Norway Maple Acer platanoides. Cappadocian Maple has simply-lobed leaves: http://www.botanicalgarden.ubc.ca/forums/showthread.php?t=9957