When do I let other places I applied to know I have an offer? I currently have a job offer, but I have had two other campus interviews.  The other search committees often mention that I should update them on my job search.  I am not sure what this means exactly.  Of course, if I know that if I take another offer, that I should let the others know. What I am unclear on is if I have an offer, but no offer yest from the other institutions, should I let them know about this?  Or do I wait until I have an offer from them to say anything?
It's in your interest to inform the other two departments that you have an offer and the date by which you need to respond to that offer.  You can say this without naming the first institution or the salary that you've been offered.  

This way, they have a chance to speed up their process and make you a competing offer before the first offer expires.  If you don't tell them, then they'll simply continue with their search process and you might end up being forced to accept the first offer before the other two institutions have completed their search process.