It's the eternal city, so there's really amazing history everywhere you turn. The Vatican is incredible (the Sistine Chapel is a masterpiece), the Colosseum is amazing, the Forum is spectacular, the Spanish Steps are pretty.
If you want something truly unique (and a little creepy), check out this place:
http://www3.sympatico.ca/tapholov/pages/bones.html
Another link:
http://eternallycool.net/?p=728
It's a tiny church on a non-descript road, easy to miss if you're not looking for it. Every piece of decoration inside the church is made from the bones of monks of the Capucin order who were displaced at some point when their original burial site was dug up. To honour their devotion to God, it was decided to literally build a church from their remains.