Well my friend, it's called a yidaki, otherwise known as a didgeridoo (a.k.a. didjeridu, didj, etc.).  Technically, it's an aerophone.  And technically, it is a big hollow log - a eucalyptus log, to be precise.

Here's the "encyclopedia" definition:
Didjeridu, musical instrument, a trumpet used by the Aborigines of northern Australia to accompany singing and dancing in rituals and entertainment. In use for about the past 1000 years, the didjeridu is made from an irregular eucalyptus branch about 1 to 1.5 m (about 3.25 to 5 ft) long and has a conical bore that is hollowed by termites. The entire length of the tube is often decorated with totemic designs. At the smaller end there is a mouthpiece fashioned of pliable beeswax. Players produce the fundamental note by loosely vibrating their lips against the mouthpiece. To avoid pausing for breath, players inhale through the nose and store air in their cheeks, a technique known as circular breathing. This technique permits the production of a continuous drone that can be altered by the lips, by the tongue, and by diaphragm pressure to vary the pitch and texture of the instrument's sound. Vocal sounds are integrated to create buzzing, growling, humming, and croaking effects that imitate birds and animals. In recent years the didjeridu has been adopted by non-Aboriginal musicians.
"Didjeridu," Microsoft® Encarta® 98 Encyclopedia. © 1993-1997 Microsoft "You Will Be Assimilated" Corporation. All rights reserved.

Here's mine: The didj is possibly the oldest musical instrument on the planet, and it's still used in sacred performances by Yolngu  tribes (the folks who invented the instrument, possibly over 30,000 years ago) in NE Arnhem land, Australia.  Its purported healing powers (I'm defining that very loosely, so don't break out the bongs just yet) are utilized by various meditation and rehabilitation clinics around the world.  While it might be a "Log" ("it's big! it's heavy! it's WOOD!"), it's also firmly rooted in mankind's cultural history.  It may in fact be the world's first "bass" instrument, and has traditional uses similar to those in which a western bass accompaniment would be employed (no wonder I like it...).

In case you're one of those auditory learner types, you can listen to a sound clip of me playing the didgeridoo pictured on the top left-hand corner of this page (made from Bloodwood eucalyptus): Joe playing the didj (mp3).  This particular didj has been hand shaved/tuned to the key of D; a didj's key is basically determined by its length and diameter, kind of like a pipe from a pipe organ (shame on you for not paying more attention in physics class!). 

Traditional yidakis are made from the trunks of eucalyptus trees that have been hollowed-out by termites (though non-traditional didgeridoos are made from all kinds of materials now, including leather and PVC pipes!). Since this is, as you'd imagine, a process steeped in a fair amount of randomness, thanks to the universal magic of chaos theory no two didjs can ever be exactly the same.  And, since your lung cavity vibrates along with the didj when you play it, no two players ever sound exactly alike, either.  Unlike traditional Western instruments, it's nearly impossible not to find your own personal "voice" on the didj.

Some (more-or-less) mainstream bands that have used the didj include Yothu Yindi, Yes, Tuatara, and Jamiroquai, while cross-over artists like Alan Dargin, Axis, and David Hudson make the didj a central part of their music.

To play the didj seriously is to owe something back to the Yolgnu culture that fathered it.  The story behind the didj is a long and complex one.  Fortunately, several other people have already told it in ways far better than I ever could, so if you want to learn more I suggest you start with the following websites:

• http://DidgeWorld.com - A comprehensive list of Didj websites
• http://www.didjeridu.com/wickedsticks/articles/articles.htm - Articles concerning the cultural significance and importance of the didj
• http://timelessproductions.com/didjeridoo/tutorial/ - David Blonski's excellent playing tutorial covering the basics of droning and rhythmic variations