Be wary of gladiators outside the colosseum. They don't take photos for free and can be quite agressive about their payment. Also don't be fooled into taking an unlicensed guided tour of the Colosseum either....

MJ Article on Colosseum

Some other suggestions, although not necessary - Travel pillow, chewing gum (especially for those who have ear problems during flight), book and/or ipod and one change of clothing (air lines do lose luggage occasionally)

scuba_Nikki Article on Carry on luggage

Cafe lattes, bruchetta, chianti, gnochi, limoncella, baci chocolates, gelati, pizza, it's impossible to not try everything in Italy.

Krista Article on Italian Food and Drink