It is reportedly possible to go to and from Hanoi's Noi Bai airport using buses 7 or 17. Also, you can take number 9, 18, or 23 and tour around the city in a circle, returning to your point o origin without backtracking, all for for only 2,500 dong, about 15 cents. The buses are nice enough, in fact some of them are the same models used in Paris.
The cheapest way of getting around Hanoi and all of Vietnam is by bus. The bus systems are well developed in Vietnam, but i'm only referring to the system, not the buses themselves (poor condition and there can be stop dead traffic, or like that when you stuck behind people on bikes al the way to HCMC). The publis bus system is so cheap and you can basicly buy a ticket from Hanoi to HCMC for about 10-20 USD. There are also tourist buses for travelers to see all of Vietnam. These buses are in good condition and are meant for tourists (and are owned by travel companies). They are a bit, but not by much, more expencive. They stop throughout all of Vietnam, you can spend a week traveling by bus spending a day in each major city. Prices vary but from HCMC to Hanoi is around 20-30 USD.
(last date took the mini-bus - April 24, 2007)
Hanoi to Haiphong
included in the package tour going Halong Bay from Hanoi