I recently picked these up at Cheat Code Central:

All Consoles:
Bonus characters
Successfully complete the indicated task to unlock the corresponding character for purchase:

Billy Corgan: Play "Today" in Band Career mode.
Hayley Williams: Play "Misery Business" in Vocals Career mode.
Ozzy Osbourne: Sing "Mr. Crowley" and the encore song at the "Germany: Ozzfest" gig in Vocals Career mode.
Rockubot: Successfully complete Drum Career mode.
Skeleton: Successfully complete Vocals Career mode.
Sting: Play "Demolition Man" in Bass Career mode.
Ted Nugent: Win Ted Nugent's Guitar Battle in Guitar Career mode.
Travis Barker: Play "Dammit" in Drum Career mode.
Zakk Wylde: Win Zakk Wylde's Guitar Battle, and play the encore song with him at the "Louisiana: Swamp Shack" gig in Guitar Career mode.

Kurt Cobain (Nirvana) custom character
Use the following customizations:

Head

Nose: Narrow larger at tip
Mouth: Top: Smallest; Bottom: Third smallest
Eyes: Little larger and deeper; Color: Hazel
Facial Fair: 5 o Clock Shadow; Black
Body

Skin: Pale
Proportions: Skinny and tall one
Presence: Calmer ones
Hair: Seattle's Finest; Color: Second lightest brown
Outfit

Torso: Layered T: Color; Default
Pants: Deez Denims; Default
Shoes: Slips Ons; Black
Instrument

Fender Mustang
Style: Treble Maker
Finishes: Custom Color: Second darkest blue row, then second darkest within
Fret Board: Ebony
Inlays: Dots
Head Stock: Style: T-bone; Color: Same as body
Pickguard: Style 1; Color: Third, gray but it looks white
Pickups: Strates Three: Black
Knobs: Speedsters
Bridge: Defender; same color as pick guard
Strings: Default[u]