If I am visiting the US with a tourist visa, can I look for a job in the US?
Contrary to the allegations of some other answers here, CBP (U.S. Customs and Border Protection) does not make up rules on the spot. Here is, quite literally, the letter of the law on who's allowed into the US and who's not:
The vast majority of that is about criminal records and whatnot, but Section 5a is key for us:
Boldface mine. In other words, if you give CBP reason to suspect that you're coming to the United States to work on this visit (say, a plumber rocking up with a box full of tools), you will be denied entry. But you are perfectly within your rights as a tourist or business visitor to enquire about future employment, go to interviews, etc, as long as a) you're not paid money for it, and b) you leave the United States and get a proper work visa before you come back.
Also, as long as your primary purpose for visiting the US is tourism, it's not a lie to state your reason for entry as "tourism" and then do a little job-hunting on the side.
Searching for a job
No you can most definitely not legally look for work while on a tourist visa.
If any evidence is found that you are looking for a job you will be denied entry on arrival or could risk deportation at any time.
Things that I've heard of US customs using to deny entry or deport travellers:
If you are randomly selected or if you fit some profile they may suspect you of looking for work and specifically look for these things.
For deportation I hear the single biggest reason is being reported by somebody. So if you do entertain the notion of looking for work even vaguely during a tourist trip, don't go around telling too many random people you meet, or don't go around giving people reasons to dislike you.
(The very same tactics apply in at least Australia and the United Kingdom by the way.)
Other sources that say you may not search for a job on a tourist visa
Attending a job interview
This seems to be treated as a separate case. I have no experience with this or anecdotes from friends or acquaintances. Please refer to Pablo's answer.
This is absolute drivel. Sad this comes up as the first post on a google search on this topic. So tell me how does one logically come to the USA for a job interview?
It is absolutely OK to travel to the USA on a Tourist visa if you intend to 'look' for work or attend interviews. If asked at the PoE, you state your purpose. I have done that twice now. They typically give you much less than the usual 6 months - say 3 weeks each in my case when I attended interviews for only 2 days. Just be honest about your intentions.
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On the official US government site "Travel.State.Gov" is relevant information
(Second part added from the same source by hippietrail, was not present in original answer.)