This topic is inspired by Ed's thread where he says the Eiffel Tower by day is overrated and it should be seen at night. I started thinking--what other European sights really are best after dark? (Subject to your own personal definition of 'best' of course!)
For me, Trevi Fountain in Rome is much more interesting and impressive when lit up at night than during the day. I also find the Pantheon to be a bit more mystical when dark.