Enter the following at the cheats screen under 'Options':

Bails 1 FMV:
Get 3 secret tapes.

Bails 2 FMV:
Get 6 secret tapes.

Always Hard FMV:
Get 9 secret tapes.

Play as Iron Man:
Beat Story mode on Beginner difficulty.

Play as Gene Simmons:
Beat Story mode on Normal difficulty.

Play as T.H.U.D.:
Beat Story mode on Sick difficulty.

Cool Specials, Kid Mode, Rollerskates, Flame, Always Special, Perfect Skitch:
Accomplish all 129 goals in the game.

Hotter than Hell level:
Beat the game with a created character.

School II Level from THPS2:
In New Jersey, go down to the subway rails area. Get a lot of speed, and launch off the middle ramp next to the tunnel. If you get high enough, you should be able to grind on a ledge on the side of the big building. Immediately go into walking mode. Start walking on the ledge and jump up to grab the spinning red icon to unlock the level.

Venice Beach level from THPS2:
In Hawaii, find the smoking tiki and jump into it. At the end of the tunnel will be the tiki again. Above it will be a spinning red icon. Nab it to unlock Venice Beach.

Hanger Level from THPS2:
In Moscow, start between the bleachers and the building near the tanks that say "AEHNH" on it. Wallride the building and grind the edge of it. The edge is lined up with a window on a big red building. Jump off the edge of the building and you will crash through the window. Once inside get off your board and go up the ramp on your right. The icon for the Hanger level will be up there.

Thanks to Revolution readers Minature Tim, CarnivorousWinds, always!, Silmir and Rob D.!