So far, seven DHARMA Initiative stations have been explored on the show. This page chronicles which survivors and Others have visited each.
The following is a table of the survivors and which station or stations they have visited. The table features survivors of the crash (without Walt, because we don't know which stations he visited when the Others kidnapped him) and Desmond, who joined the Losties. Danielle is also listed here, though she may have knowledge of DHARMA stations that has been unrevealed.
By the end of Season 2, most of the main characters have been shown inside the station on the show. Also, it is taken as common knowledge that all the other survivors know about the Swan station due to Locke's statement at one time that the rotation list for the button had "everyone" on it. (2x09)
Desmond predicted that Charlie had to go here to flip the yellow switch so they could get rescued. He said Charlie would swim down and drown. Desmond offer to go instead but Charlie knocked him out with a paddle.
Charlie swam down to the station to find it not to be flooded. After being briefly held hostage, he completed his mission, and Mikhail detonated a grenade from outside one of the portholes, flooding the communications room. (3x21) (3x22)
The most experienced travelers in terms of DHARMA stations are:
Kate could be considered the most experienced survivor, because she has visited the most stations out of all of the survivors. The only three stations she has not visited are the Pearl, the Arrow, and the Looking Glass. Visiting the Arrow station, however, would give her little, if any, additional knowledge of The Others, because she is already aware that they wear costumes. She is also one of the few survivors to have seen The Others' Barracks.
Locke is an experienced survivor. He did briefly see the blast door map, meaning he has potentially more knowledge than all the other explorers combined (though the fact remains he remembers very little of what was drawn on the door and was later more interested in running the timer down to zero than seeing more of the map. While he has been exposed qualitatively to more information than any other survivor, his mystical attitude toward the Island leads to him taking little advantage of the information. When presented with a wealth of new information at the Flame, his response was to destroy everything.
Sayid is also reasonably experienced, having explored the Swan, the Pearl and the Flame in much detail - searching the underground tunnels of the Swan, checking out the electronics of the Pearl and looking through some of the files at the Flame. But he shows little interest in gathering information outside his immediate personal objective.
As well, counting "non-Others", Kelvin and Radzinsky's exploring led them to several of the stations identified on blast door map. They appear to have been to the Swan, the Staff, the Arrow and the Flame. The map indicates they did not reach the Pearl but knew the station existed.
The following is a table of the Others and which station or stations they have visited. The table lists all Others who have been known to visit at least one station. Due to the fact that the show has not followed the Others as thoroughly as the survivors, there may be Others who have visited stations off-screen, despite the fact they are listed as 'not visited'. It is also harder to note which stations each Other does know about - thus, each Other will only be listed as either 'Visited' or 'Not visited' each station.
Juliet took Sun to the Staff in order to determine a D.O.C. (Date of Conception) for her unborn baby. The Staff was also where Juliet left her recorded messages for Ben whilst she infiltrated the survivors. (3x18)
Ethan, Goodwin and Sabine were seen in the operating room in Juliet's flashback. (3x16)
Ben and Juliet have visited the most DHARMA stations, having been in 5 each as of the Tempest. They may have at some point also visited the other stations, but it has not yet been shown. Following them is Tom, who has been in three stations.