By songthaew or seelor
The most common way to get around Chiang Mai is by songthaew, also known more locally as rod-daeng or "seelor". These are covered pick-up trucks with two benches in the back, and indeed songthaew means "two benches" in Thai. Seen everywhere, to board one just put your arm out and look at the driver, who will stop. Then tell the driver which street you want to go to, and if he is going that way, he will nod his head "yes"; if not, he will say "no" and go on. Don't worry - there will be another one right behind him. When the driver turns down the street you want, start looking for where you want to get off and press the switch located on the roof of the cab. The driver will pull over, let you out, and then you pay him. The fare should be 15 baht for regular trips around town. If you specify a hotel or establishment, the driver will think you want to hire him for a private trip, and the price will be much higher. Negotiate any price beforehand if you want to go to a specific address. The best way to avoid this "charter" situation is to discuss your destination and not the price; asking for a price is interpreted as asking for a charter. Then, when you arrive, hand the driver the correct change. If the driver demands more, then it is up to you to work out a fair payment, but armed with this information, you should have a reasonable idea of the proper fare, and that will aid you in your bargaining.
The colour of the songthaew indicates its general route or usage. Most common by far are red songthaews (hence the alternative rod daeng, "red car" name), which roam the main streets in the city itself. Warorot Market (by the Ping River) is the most common terminus for songthaews that travel along fixed routes. From Warorot Market, white songthaews travel to the eastern suburban city of Sankampaeng, yellow songthaews travel to Mae Rim in the north, blue songthaews travel to Sarapee and Lamphun in the south, and green songthaews travel to Mae Jo in the north-east. The flat rate cost along these fixed routes is 10-20 baht.
From Pratu Chiang Mai market, songthaews also travel to Hang Dong (20 baht) and San Patong, south-west of Chiang Mai.
You may see songthaews out on the highways in the countryside, travelling to and from small towns and villages. It is probably not proper for them to do this (as such travel is supposed to be done by bus companies), but in Thailand people will find a way to make some extra money.
By tuk-tuk or samlor
Tuk-tuks serve as Chiang Mai's taxis, going point to point for 30 baht and up depending on your haggling skills. A few three-wheeled bicycles (samlor) still cruise the streets and will go your way for the same price, which is a great way to see the inner city temples. Try taking a samlor from Wat Prasing Temple to Wat Chedi Luang Temple in the early evening around sunset, or around the inner city at sunrise to see the monks walking around with their bowls collecting alms from the citizens.
Chiang Mai has finally introduced Bangkok-style metered taxis. In early 2005 there were only 15 plying the streets (versus 2700 songthaews), but one year later there were over 45, with the number growing monthly. Rates are very reasonable at 30 baht for the first 2km and 4 baht/km after that, however it's very hard to persuade the driver to use the meter. Dial +66 53-279291 for advance bookings, which are particularly useful when going to the airport (100 baht flat fare).
Chiang Mai's on-again, off-again local bus service began operation again in November 2005. There are currently 5 routes and fares are a flat 15 baht. Route 4, connecting to the airport, is probably the most useful. See Chiang Mai Bus for a route map.
Chiang Mai has an abundance of motorcycle rental services, with choices aplenty. Typical Asian motorbikes can be rented, such as Honda and Yamaha 110cc and 125cc models (both step-through and automatic), but off-road bikes and larger street bikes can also be found quite easily. Renting a small bike starts at around 200 baht/day with insurance; larger machines can climb to 800 baht/day for a V-twin chopper or large sport-bike, also with insurance. Expect discounts when renting for several days. Passports are usually taken as a deposit, although some shops will accept a photocopy with a cash deposit of a few thousand baht. As with any other tourist town in Thailand (except possibly in Bangkok), a valid international permit (IDP) isn't required by the rental shop; however, if you find yourself stopped by the police for whatever reason, be prepared to discreetly pay a small "mai pen rai" fee of at least 200 baht.
Within the old city walls biking is still an easy option to get around. You can get everywhere in town within 10 minutes and it saves the hassle of negotiating with tuk-tuk drivers all the time. Bikes rentals are offered at every other street corner, and for a simple bike start from 30 baht/day.