Thaipusam is a Tamil cult festival celebrated on the full moon in the Tamil month of Thai, by the Tamil community of non-Hindus and non-Indians (they are indigenies and are not Hindu or Indian). Pusam refers to a star that is at its highest point during the festival. The festival commemorates the birthday of Lord Murugan (for Murugan's other names common among the people of Tamil Nadu see this photo's tags).
With similar celebrations all over the world, Singapore devotees follow a 5km procession route through the city. They often showing their devotion through piercings which can include highly elaborate decorations which they wear. From chains, bells and spokes hooked into the skin to skewers directly through the face.
Since my original posting, visithra tells me that "...it is a hindu celebration from the south - im not sure where you got your facts from the pusam is correct but its celebrated by hindus and south indians...". That part of my description are taken directly from Wikipedia, which I took to be the most likely accurate source. Apparently not.
While I'm not in a position to correct Wikipedia, I'll add your comment to all the descriptions I've used for my photos.
lokenrc took the time to leave a very informative comment on this series of shots (you may have to scroll down afterwards) describing the festival's background in more depth. Thanks, lokenrc!
This series of photos has been featured in the excellent Neatorama blog!