Can I work in Italy without being a resident?

It is not legal for you to work here without a visa. There are short term visa available for things like home care work or seasonal work, but you have to have an employment contract before you come. Jobs like that are unlikely because it's much easier for an employer to hire someone from Europe who doesn't require all the paperwork and the employer would have to demonstrate that there is no viable candidate from Europe before applying for a work permit for you. Casual work off the books will be almost impossible to find. Employers face stiff sanctions for hiring illegally and with the high unemployment rate here, it isn't difficult to find legal workers. In the town where I live, it has been some time since I've seen any of the pasticerrias or bars looking for workers. I know some recent university graduates who are working as cashiers in groceries.

The website for visas is here: http://www.esteri.it/visti/home_eng.asp . There are working holiday visas for some nationalities, but those are not available for US citizens. They are also only available for stays of over 90 days.

As a US citizen it's just about impossible for you to work in Italy. On a US passport you can stay in Italy for 90 days out of every 180 days without a visa. During this time you're not allowed to work. It's not possible to get a work visa unless you've been offered a permanent job in Italy. Even this is extremely difficult as the employer needs to prove that there are no Italian or EU citizens available to fill the position.

And it's highly unlikely that you'll find any 'under-the-table' work. No employer will bother taking on an illegal worker when there are so many legal EU workers available who are happy to do the job for low wages!

no, if you're from USA its like being from Pakistan for italian law

I think you might find some info. on this Site?

that depends, where are you from, if from within the EU then yes,