Getting there and away

Air

Siem Reap's international airport is 7 km out of Siem Reap proper. A US$25 departure tax for international flights and a US$6 departure tax for domestic flights applies.
http://www.cambodia-airports.com/siemreap/en/

Means for getting from Siem Reap International Airport into Siem Reap include:
Taxi
Taxis can be picked up from a booth at the airport. The fee is US$5. Bear in mind the driver will hope to garner your custom for the duration of your stay, taking you around Angkor and so on -- this is totally up to you.

Motorbike
A moto from the airport into Siem Reap should cost US$1.00-1.50. You can often get the ride effectively for free by agreeing to hire the moto to take you around the ruins or show you to a guesthouse where they can get a commission for your stay. It is extremely bad form to negotiate a free ride and then decide not to use the moto for the ruins after all.

Bus

The main bus station is situated outside Siem Reap at the taxi-park though many bus companies will offer to pick you up and ferry you out to the bus station. The best company doing the Siem Reap to Phnom Penh run is Mekong Express, which costs US$8 and takes around four hours. GST and Sorya buses take about six hours but only cost $5. All buses break the journey for a snack at Kompong Thom. The Mekong Express buses have a toilet on board -- other (cheaper) services often do not.

Boat

If you're arriving by boat from either Phnom Penh or Battambang, you'll disembark at Chong Khmeas ferry dock, some 12km south of Siem Reap. The trip takes about 30 minutes. The boat to Phnom Penh is a speed boat, costs $35 and takes around five to six hours. Transport to Siem Reap should be included in the price. The boat to Battambang is a slow boat and can take anywhere from four to 12 hours, depending on the time of year and current, and how many times you stop. Expect a moto to charge around US$2 to get into town while a car should cost about US$6.

Other

The easiest and fastest way to get to the Thai border at Poipet is to take a share taxi from Siem Reap. You can either hire the entire car (US$30-35) or buy one (or two) seats in a car. Do not, under any circumstances, take an organised minibus to Bangkok. See Tales of Asia for detailed coverage of this route.

Getting Around

Remorque motos
Best described as a motorcycle towing a chariot, remorque-motos can be found on just about every street corner. Short hops around town shouldn't cost more than a dollar, but if you're planning on using these frequently, hire one for the day at the universal price of $10. They can comfortably seat two people and three or four at a squeeze.

Moto
Short hops around town shouldn't cost more than a few thousand riel, with daily hiring starting at about US$6 depending on where you want to go. Bear in mind the driver will not have a helmet, for himself or you. You can pick motos up anywhere -- just look for a Khmer guy wearing a baseball cap waving at you.

Foreign tourists cannot hire motorcycles in Siem Reap.

Taxis
More expensive but more comfortable than other options, figure on US$25-30 per day for a Toyota Camry, more for a minibus. If you're planning on visiting outlying ruins and have a few people to split the fare with, this can be a smart way to get there. Most guesthouses and travel agents will be able to sort out a car for you, or just ask a moto and he'll find you one.