On foot
Personally, I'd like to walk around cities I visit, but the tropical heat in Phnom Penh can be a drawback. Still, large parts of Phnom Penh can easily be explored on foot.
Bicycle
Another healthy option is a bicycle. Ideal for short journeys and available for rent with prices starting at as little as US$ 1 per day. Just be careful negotiating the traffic, which can be challenging to say the least. And don't forget to lock your bike!
Cyclo
Last of the non-motorised options is a very common sight in Phnom Penh. The cyclo is a three-wheeled bicycle, with the driver sitting above and behind the passengers.
It should have a shade to keep you out of the direct sunlight, and side flaps to offer protection from the rains (especially in the rainy season). Price: around 2000 riel for a short journey.
Motodop
Probably the most common means of getting around Phnom Penh is by motorcycle taxi, called 'motodop' in Khmer.
Any baseball-hat wearing driver is a motodop and there are thousands of them! A short journey should cost you no more than 1500 riel, but be sure to agree the fare in advance. Prices go up a little at night.
Tuk-tuk
Ideal for groups of 2-4 people is the tuk-tuk. A little different from the Thai version, in Phnom Penh it being a kind of hybrid moto-chariot.
Tuk-tuks have the benefit of offering some protection from the elements, whether the fierce tropical sun or the rains. Tuk-tuks are plentiful at the riverfront. Expect to pay US$ 12-15 for a full day.
Taxi
And then there is the taxi, typically a white Toyota Camry for some reason. These can be arranged through your guesthouse or hotel and should cost about US$25 per day.
Some negotiating skills are needed!
For those of you looking for your own motorised transport, there are several motorbike rental shops across the city. You can hire a simple step-through moto (usually a Daelim or a Honda) or a 250cc dirtbike if you are a little more experienced. You will be expected to leave your passport as a deposit.
It's worth checking the bike's condition before you set off, especially if you're planning a long journey. The quality of the bikes vary from place to place, though prices tend to be similar. For a step-through US$ 3 a day, for a dirtbike around US$ 8.
If you are willing to spend more, try the Phnom Penh Bike Shop at #31 Street 302. The prices are a little higher, but the bikes are very well maintained and come with insurance.
All rental bikes should come with some form of security, if only a Solex padlock. It's well worth using whatever you are given, as having a rental bike stolen can really be a painful and expensive experience!