Although a little temperamental - I too would suggest African Violets as being a very good choice especially as there are so many different varieties to choose from
However there is one plant that I would highly recommend as being an excellent choice for a shady windowsill and that is Porphyrocoma Pohliana commonly known as
The Rose Pine Cone Plant
which apart from being a very hardy easy care plant is very adaptable too - as although it prefers a shady spot - it will grow in almost every type of light too and as it flowers continually throughout the year - it is in my mind what makes it an ideal plant for a shady windowsill - as not only does it flower continually throughout the year but has year round striking foliage too for very little effort
Orchids are truly gorgeous! I didn't know that they grow well in shade.
Another interesting, flowering plant that doesn't need much sunlight is Thanksgiving cactus. I've never been a big fan of cacti, but in my opinion this one looks quite pretty: