Most AFAIK. My understanding (IANAL) is that, in Massachusetts for example, it's not that there are laws explicitly allowing non-competes on the books but, rather, that they're supported by common law spelled out through many precedents related to contract law, employment law, etc. I imagine the situation in many other states is similar. California is unusual in that they have specifically limited non-competes by statute.

Georgia passed making non-competes enforceable by referendum a few years ago. It passed overwhelmingly and many people attribute that to the ballot item being worded in a stunningly dishonest way.

http://ballotpedia.org/Georgia_Employment_Contract_Enforceme...

Several.

However the strength and scope of these non-competes varies widely. And many companies have clumsily written non-competes in their employee contracts that would never hold up under scrutiny.

NY