coleus! especially yellow or red coleus for a splash of color! (i have thought of but never tried putting these in my own north-facing window boxes). also at my local nursery i have seen these cascading annuals with tiny masses of purple flowers (don't remember the name) that are also for shade... if i find the name, i will post it here.

Fuschia! If you don't mind cleaning up after them, they are beautiful , exotic looking and come in lots of colors.
The New Guinea impatiens are great too, with huge flowers and more colorful foliage. Annual variegated vinca also does well in my containers on the north side of my house.