As posed on the Thai Visa Forum last September: What Isp To Go For Here In Chiang Mai ?
Wikipedia does have an entry for Internet in Thailand, if you care for an overview of the nation.
Check out KIRZ (King International Resource Zone), “the commercial Internet Service Provider in Thailand.” They have a Chiang Mai office.
One Stop Chiang Mai has a brief, somewhat helpful article called Getting connected to phone and internet.
Did you know that there’s a Chiang Mai Computer Club forum?