Sai Tai Mai Bus Station
The new Southern bus station, Sai Tai Mai, is a kind of shopping mall with intra- and inter-city busses departing as well. It's easy enough to find your way around, but can be harder to find your way there. Make sure you aren't taken to the old Sai Tai Mai, which is about 10km further in towards the central city. Another note, tickets are on the "1st floor", which is actually the second floor above ground level for some inexplicable reason. The ground floor is "G", the next is "M" and then you get "1". Huh.
There is a bus running between here, Khao San Road and Suvanabhumi Airport, which is damn convenient for backpackers, and I believe the number 526, but you'll have to look out for it - exciting destinations (e.g. those mentioned before) are all marked in English.
Busses here mainly depart for west and southern destinations.