For me, this is post number 666, so of course, I will devote it to highways with that number!
I know, of course, that US 666 in Colorado and New Mexico was famously renumbered to US 491 a while back, because of sign theft, I believe. As for state highways with the "number of the beast", I know that Pennsylvania and Ohio have them - what other states might? In Virginia, "secondary" state highways are numbered 600 and above and are repeated from county to county - I'm sure that at least one county in that state has a VA SR 666. What about the 600-series county roads in New Jersey, or the 600-series band of state highways in Florida, which includes the Tampa area? Also, is there an FM 666 in Texas?
In my opinion, County Road D-32 in Michigan, near Pinckney in Livingston County just south of Howell, should be designated as M-666, as it is quite literally the "road to Hell" in that state - although I'm sure MDOT would have a huge headache on their hands with regards to stolen signs. How much of a problem is it for the states that already have a Highway 666 - or, for that matter, Mile Marker 666 on I-10 in Texas, between San Antonio and Houston? I'm interested to find out more.