If Mars Volta defines prog rock for the last decade, Tool defines 1990s prog. The group probably can be more accurately defined as prog-metal, and they retain a gigantic cult following despite their minimal output. The group has been together for 21 years, but in that time they've only released four LPs. They don't have anything that even resembles a hit single, but they pack arenas and headline festivals every time they hit the road. The band toured last year and were talking about a new LP, but the group's leader Maynard Kennean is devoting much of this year to his side project A Perfect Circle.