Cambodia Angkor Airways offer several daily flights to/from Phnom Penh, another cheap opportunity is Bassaca Air or Bayon Airlines. Vietnam Airline also offer daily flight to/from Ho Chi Minh city and Hanoi, Singapore Airlines to/from Singapore and Loas Airlines to/from Luang Prabang.
BY RIVER FERRY
Daily ferries ply the Tonle Sap river and lake between Phnom Penh and Siem Reap. The end of the trip is marked by a hill, Phnom Krom, near the ferry dock at Chong Khneas 12 km south of Siem Reap. During the dry season, the ferry stops short of the dock and passengers transfer to smaller boats to traverse the final few hundred meters.
Ferries depart 7am daily from the Phnom Penh Port on Sisowath Quay. Ferries depart Siem Reap daily at 7am from the dock at Chong Khneas. Passage is around $18-$25 and should be purchased a day in advance (251km, 4-6 hours). Drinks are sometimes available. Tickets can be purchased through hotels and travel agencies cheaper than at the ferry offices. Though generally safe, these ferries are local transport and have experienced breakdowns, groundings and other difficulties. Travel is best during the wet season (June-November). Dry season low waters can mean smaller, less comfortable boats and occasional groundings.
There are many Bus companies like Giant Ibis, Mekong Express, PSD Xpress, Sorya Bus, Neak Krohorm, Thero Express (15-seat), Larryta, KSO, Virak Buntham, Kampot Express, Cambodia Post VIP Van offering bus/van services to Siem Reap town using modern air-conditioned buses/vans. Buses/vans depart every 15 minutes to one hour, daily from 6am-12pm mid night. The prices are reasonable.