I consider "no traffic" to be a big part of a city's attractions. Having experienced Manila's traffic hell, Jakarta's traffic's jams, and having commuted in traffic 2.5 hours every morning, I really enjoyed the virtually nonexistent traffic in Siem Reap. But the city is too small to warrant serious traffic jams, which is why I like it even more.
Many people know Siem Reap is the base town for tourists who visit the ancient temple of Angkor Wat in Cambodia. The town is so tiny you can actually explore it on foot, not very comfortably and quickly, perhaps, but still doable. Even so, there are several transportation choices that won't ever become an issue. Here are some.
Foot
What better way to really experience a new place than to walk around leisurely, observing every little thing that could otherwise be missed? You'll get to see the Khmer people up close, see how they trade, how they eat, how they interact with each other, and see how nice, or not nice, the infrastructures are. Cost: $0.
Bicycle
In Siem Reap people use bicycles often, and for good reason too: the terrain is pretty flat! For in town purposes, a bicycle is a good choice. Apart from walking, this is another great way to experience the small town. It's also easy to rent one; most guest houses and other accommodations offer this service.
Cost: $2.00 to $4.00 per day.
Motodop/motorcycle
No helmet necessary, just hop on the back of the driver and tell him where to go. Within five minutes, you'll most likely arrive at your destination, wondering why you paid $1.00 for such a short ride. In Siem Reap, foreigners aren't allowed to rent motorcycles, probably because motodop services provide employment to the locals. Cost: 2000 Riel – $1,00 for short trips around town. $8.00 to $10.00 a day around the Angkor area.
Tuk tuk
This is the budget-luxury mode of transportation. You can sit back on cushy cushions, be protected from the sun, wind and rain, and still be in close contact with the surroundings. Tuk tuks are motorcycles pulling a carriage that can fit up to four people comfortably. It is probably the de facto form of transportation there. When you see an almost endless line of tuk tuks at the foot of Phnom Bakheng, you would think the same, and wonder about the state of Siem Reap tourism industry. Cost: $1.00 for short trips around town. $12.00 to $20.00 a day for the Angkor area.
Car/taxi
Cars and taxis are the luxury transportation. Almost all are equipped with air conditioning, a lifesaver if you're visiting Angkor Wat during the dry season. Going around by taxi is somewhat a sterile experience because you breathe none of that natural Cambodian air, but great for visiting faraway temples.
Some Angkor temples like Beng Mealea or Phnom Kulen are located further from the Angkor Archaeological Park complex, 50 to 100 kilometers away. When you're in a tuk tuk for that long, you'll start to like it less and less. The wind will be too strong, the sun too bright, the dust too invasive, the roads too bumpy. All that you previously thought were wonderful about tuk tuks now become horrible. Save yourself the annoyance and use a taxi for faraway temples. You'll be glad you did. Cost: $5.00 for short trips around town, $25.00 to $30.00 for the Angkor area, $30.00 to $60.00 and up for more remote temples.
Van
A van is only useful if you have a large group, more than four people. It is similar to a taxi in terms of what it provides (air conditioning, good interior, etc), but it's also pricier. Then again, if you decide to rent a van, most likely it will come out cheaper per head.
Check out Angkor Wat resource website for more tips and photos.