You should have no problems hooking up to some free wifi in Chiang Mai. In some parts of the city it's difficult to go 50 metres without stumbling on a bunch of signals.
The densest concentration is probably in the area popular with trendy locals, Nimmanhaemin Road. It seems like every coffee shop and smart restaurant down here provides wifi.
Up in the backpacker part of town you might struggle a little bit, but you can combine surfing and drinking in such pleasant establishments as the Pirate Cove on Ratchamankha Road and Chiang Mai Saloon on Rajvithi Road. There are also a few smart coffee shops such as Wawee's to consider.
For the more serious Internet junkie you'll be looking at rooms equipped with wifi. For this I'll direct you to 'wifi alley', Ratchadamneon soi 1, just south of Thapae Gate. You have the very comfortable Ban Wiang Guesthouse along with the slightly more upscale Boonthavon and Mini Cost. At the end of this alley there's even the more backpacker level Rama Guesthouse sporting wifi! And don't forget the Montri right by Thapae gate - their signal seems to reach just about every cafe in the vicinity.
Others: The Garden Chiang Mai on Loi Kroh Rd, Prince on Tai Wang Rd, Viang Bua on Viangbua Rd and Galare Guesthouse on Chareonprathet Rd.
Of course, there'll be more when you get there.
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