No one mentions Prosecco (http://prosecco.us.com/) much but I have always liked it. It's what they call "upscale comfort food from all 20 regions of Italy". Located in River North. It's definitely more of a formal/classy ambiance. I have always had great service both at the bar and tables. The complementary glass of prosecco at the beginning of your meal is a nice touch. I find it pleasantly different from the typical american-italian restaurants you can find almost anywhere, great food and service.
For a very different flavor I love Davanti Enoteca (http://www.davantichicago.com/) although I guess it is not technically downtown as it's on Taylor St near UIC. One of my favorites.
There's a couple of good options not typically mentioned in the Italian food threads.
I just had my BD dinner with the wifey at Ruxbin. Excellent dinner. Definitely in my top10. They do not take reservations and from what I read on a typical night the seating wait list fills up for the evaning by 7p. They open at 530 and Sunday is the best bet - luckily my birthday fell on a Sunday and we got there right at 530 and got a table right away. 20min later and we'd have been sitting upstairs drinking wine while we waited (BYOB). As long as you know what to expect as far as the wait and such, the dinner and service were amazing.
Living in Old Town I immediately thought of:
The Spice House (http://www.thespicehouse.com) for truffles (they grind up their own truffles fresh for their truffle salt... truly amazing)
and Old Town Oil (http://www.oldtownoil.com) next door for truffle oil
...as GenkiTango said - it's easy to make yourself. By the way - highly recommend these shops for all sorts of yum.
Old Town Oil