The beacon was lit by the Others or by Rousseau. The goal of this Pillar of smoke was to distract Sayid and get him as far away from the Swan as possible. It worked and he was not able to continue his push that the Hatch 'not' be opened as he was the only leading character who insisted it not be opened but rather buried. The Others wanted Locke to get inside the hatch and thus, either through direct command (Rousseau being one of their agents or members and being ordered to light it) or through manipulative coercion (they lit the fire and knew she would take the baby in futile hopes of an exchange for her own, also knowing that Sayid would pursue). Either way, the actual person who lit the fire is irrelevant, the fire was a product of the others and more directly of Ben's keen ability to get what he wants.
The beacon was lit by the real Henry Gale. He mentioned that he was going to light a rescue fire in the note he wrote on the $20 bill.
Would he have gone to the effort of constructing a brassier like the one Sayid and Charlie found? The structure looked far better built and permanent than a simple signal fire - like the one the survivors themselves built, for instance.
He may have wanted to construct a signal fire that would generate a lot of smoke and burn unattended for a long period of time without risk of being washed away, being rained out, or spreading to surrounding areas. A man traveling solo in a hot air balloon is likely to have some knowledge of survival strategies.
May be an allusion to the "pillar of smoke" that led the Hebrews through the desert in the Book of Exodus.
Remember, Sayid indicated that it was odd that there were no footprints around to have started the 'pillar of smoke"
We have seen the Others use flaming arrows in Season 5. It is possible that once the footprints were removed, they were able to light the fire from a distance.
But then the Others apparently only leave footprints when they want to.
The fire had been started very close to the water's edge and had been burning, judging by the distance from the survivors' camp, for several hours. The footprints could have been washed away by the tide.
If this had been a possible explanation, Sayid would have thought of it and would not have called it odd.
Rousseau never admits to having lit the fire. The black smoke could be The Monster performing some task. In the second season, Sayid supposedly lights a fire, but he may have never lit it (as we never see him do it). This could be the infamous unknown occurrence of The Monster, alleged by the creators to have happened before the season was out.
The Monster did light a fire when it was haunting Mr. Eko as Yemi in the tent, there were no flames in the tent until Yemi lit a brass zippo lighter.
More than likely, Rousseau did not light the fire. If she did, she wold know that Sayid would head for it, and therefore she would head somewhere else to get away with Aaron - but she headed for the smoke, meaning that she actually thought that they lit it.
The pillar of smoke that Danielle claims to have seem was actually the smoke monster. She perceived it as a pillar of smoke so she lit it in 2004 from her memory of it.