Mastodon is a relatively new metal band (formed in 2000) out of Atlanta, Georgia. dubs them as progressive metal, and they occupy the slot for 10th most popular progressive metal band. The four band members are Brent Hinds (lead vocalist, guitarist), Troy Sanders (vocalist, bassist), Bill Kelliher (guitarist), and Brann Dailor (drummer). Brent and Troy knew each other already, and met the other two at a High On Fire show that Brent was holding. Brann and Bill had met in New York and both moved to Atlanta three weeks prior to meeting the other two.
The best thing about them is that Mastodon is the perfect name for their sound. When I listen to one of their songs, I am suddenly aware that I'm experiencing the exact auditory equivalent of a mastodon in various moods and motions. Every mastodon documentary should have a purely Mastodon soundtrack. Every twang, squeal, whine, thump, burst, and crash is flawlessly metaphorical in representing thick, shaggy, resilient fur, thunderous, crushing, yet ponderous footfalls, and those solemn, spear-like doom-tusks. Their song "March of the Fire Ants," my personal favourite, is the last thing you hear before a mastodon flattens your puny body with utter indifference and smugness.
If you like thunderheads of guitar plus bass plus drums being complex and melodious while crushing you to death, listen to Mastodon. If you like progressive metal listen to them. If you like mastodons or mammoths (or maybe, and this is probably stretching it, elephants) listen to them. If you like gentle singing lightly placed over a meandering clarinet sonata do not listen to them, because you probably won't like them. Their popularity has led to accusations of them being trendy hipster metal or "mall-core", but those who say that are likely the same people who discern true black metal from false. In other words, their opinions mean nothing. The truth is Mastodon is very popular, but rightfully so.
Here's a discography of their works thus far:
Lifesblood EP (2000)
Remission (2003)
Leviathan (2004)
Blood Mountain (2006)
Crack The Skye (2009)

Lifesblood and Remission do a good job of establishing the band's trademark sound, but they really hit their stride with Leviathan, a concept album based on Moby-Dick. While the album does not exactly match the epic proportions of Melville's great American novel, the story translates well into metal. Blood Mountain is another concept album, though it is an original story by the band. It tells of a journey to the peak of a dangerous mountain filled with mystical creatures. The aim of the mission is to place a crystal skull at the top of the mountain. The inventive tale actually makes little sense but is exciting, detailed, and again, fits the music very well. Crack The Skye is based on an equally strange story, this time involving wormholes and alternate dimensions, and tsarist Russia. Mastodon has quickly established an idiosyncratic style of lyrical storytelling, and Crack The Skye doesn't seem to break the trend.
Mastodon had the honour of performing the opening theme song for the ATHF movie. The song, Cut You Up With A Linoleum Knife, was written specially for the movie and is available on its official soundtrack.