Grunge ruled the early 1990s and nu-metal broke out later in the decade, but heavy rock music has been struggling to find its identity in the new millennium, though there were several bright spots in 2005.

Green Day shined as they dominated the airwaves and filled stadiums on the success of their latest release, “American Idiot.” Ten million copies sold, a Grammy and countless MTV Video Music Awards placed them on the throne as rock kings of 2005.

System of a Down also turned some heads as they pounded out a double-album released separately as “Mezmerize” and “Hypnotize.” Despite clocking in at only 76 minutes combined, “Mezmerize/Hynotize” is easily the greatest “double-album that’s not really a double-album” since 1991’s “Use Your Illusion I & II” by Guns ‘n’ Roses.

Nine Inch Nails, Weezer, Mudvayne and Audioslave all had major-label releases last year that should not be brushed aside. Efforts by Korn, Staind, Disturbed and Sevendust fell short of expectations, but still offer a few memorable tracks.

2006 holds the potential to see rock flourish even more as some of the heavy hitters of the last decade such as Tool, Metallica and Rob Zombie have been holed up in the studio crafting new music. Here are some of the bands to look out for in the year ahead:

Tool

Depite being the most respected and talented band in rock, rumors have swirled that frontman Maynard James Keenan’s involvement with A Perfect Circle would be Tool’s downfall. However, the band entered the studio in August to put together the follow-up to 2001’s “Lateralus.”

Tool has been known to take four to five years to produce new material, but fans know it is worth the wait. Look for a new Tool around the middle of the year.

Metallica

The Bay Area Bashers have not announced plans to release a new album this year, but they spent some time in late 2005 in their headquarters/studio gearing up for a follow-up to 2003’s “St. Anger.”

In a video update on their Web site, bassist Rob Trujillo said they have been “laying out the blueprint for a new album and fiddling with ideas”; to which drummer Lars Ulrich jokingly responded, “Yeah, the next Metallica album will have a lot of fiddle on it” — perhaps a shot at the critics who didn’t like the experimental direction they took with “St. Anger.”

Metallica aren’t known for getting in a hurry in the studio, but the fans who didn’t jump off the bandwagon after the Napster mess and their supposed “sell-out” should keep their ears open for news on some fresh “metal up your ass.”

Rob Zombie

Zombie has been busy making horror movies “House of 1,000 Corpses” and “The Devil’s Rejects,” so he can be excused for not putting out any new studio work since 2001’s “The Sinister Urge.” He told Revolver magazine playing Ozzfest renewed his enthusiasm for music and he hopes to have a new album out in March.

For the new songs, he has been working with Limp Bizkit’s Wes Borland, Motley Crue’s Tommy Lee, A Perfect Circle’s Josh Freese, and former Nine Inch Nails contributor Danny Lohner, so there’s no telling what they will sound like with that mixed bag of characters. Zombie promises “more blood-and-guts headbanging metal.”

Shadows Fall

I’m sure there is a wide array of underground bands out there making great music, but I don’t usually take the time to wade through the crap to find them. However, when I saw little-known band Shadows Fall open up for Slipknot, their performance forced me to give them a chance and now I am convinced they are the future of speed metal.

Download “Deadworld,” “Thoughts Without Words,” or “Inspiration on Demand” and I’m certain you’ll agree. Their latest album, “The War Within,” showed great promise, but seemed to lack focus at times.

Singer Brian Fair admits, “Before, we’d combine a million riffs into these seven-minute songs that didn’t have a great flow.” For their new album due late this summer, Fair says, “Now, it’s more about getting those riffs into a real structure that keeps the listener interested.”

Army of Anyone

Filter singer Richard Patrick has been working with former Stone Temple Pilots members Robert and Dean DeLeo and they plan to release an album this year.

The whole “supergroup” thing has worked recently for Velvet Revolver and Audioslave, but Patrick has produced a lot of garbage lately. Don’t get your hopes too high.

Other bands with plans for a 2006 release:

Deftones, Godsmack, Ministry, Slayer, Stone Sour, AC/DC, Type O Negative, Powerman 5000 and a million pansy bands that want to be like My Chemical Romance.