Rome at night is an entirely different city to Rome during the day. I know this can be said for nearly every city but I noticed it far more in Rome. For one the churches hide in the shadows and give over the spotlight to Pizzerias and Pubs and the romantic lighting of monuments.
A trip around Rome at night should not miss the Colosseum which becomes even more impressive with its well placed mood lighting. My camera doesn't have the best night shot settings (or I haven't found them) and I don't have a tripod to still the shakes for slower shutter speeds but I really like the photo of the Colosseum above.
A trip along the river during the day shows a fairly dirty Tiber River with boring banks but at night the river edge comes alive with bars and open air theatres. I can imagine spending a good many hours exploring the riverside (and the small stalls that sprung up between bars).
The Trevi fountain was actually very boring when we visited at night, something I was really looking forward to. It looks nothing like the beautiful postcard shots.
The last photo is of something that I actually never saw during the day, the Pyramid. The photo was taken at dusk and really reminds me of a Stargate landing platform (I know I'm a geek).