do i need a Visa or a Passaport to travel from U.S to Mexico?

only if you want to come back into the u.s....the rules have changed since 9-11 dammit, you are now required to have a passport for canada and mexico and any traveloutside of the u.s.

AIR TRAVEL

ALL PERSONS traveling by air outside of the United States are required to present a passport or other valid travel document to enter or re-enter the United States.
LAND AND SEA TRAVEL
The following summarizes information available on the Department of Homeland Security’s website.

* CURRENTLY:
U.S. citizens need to present either (a) a passport, passport card (available in spring 2008), or WHTI-compliant document; or (b) a government-issued photo ID, such as a driver’s license, along with proof of citizenship, such as a birth certificate.
* LATER:
On June 1, 2009, the U.S. government will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have a passport, passport card, or WHTI-compliant document.

You do not need a visa or passport to travel TO Mexico FROM the US but getting back in you need proof of citizenship and soon you will need a passport. Not sure when that is going to take effect. Proof of citizenship used to be a birth certificate and/or drivers license, Green Card, official picture ID of some kind.
I would check the state department web site below since some of this changes day to day.

hallo,
Mexico visa is not required for citizens of United States for a stay up to 180 days.

Starting January 23rd, 2007 all United States citizens must be in possession of a valid US passport to enter Mexico. United States legal residents (Green Card holders) need their original Green Card and a passport from their country of origin.