Bill Blasters were introduced in Super Mario Bros., commonly used as defense emplacements in fortresses and castles. They are simple cannons that fire Bullet Bills while being invulnerable to attacks. Most iterations are immobile, while some others, introduced in later games, can rotate or move up and down. Larger variations which shoot Banzai Bills also exist.
Bill Blasters are usually indestructible, but they can be defeated in the Paper Mario games, where they have low-average HP but exceptionally tough defenses. As a result, Mario has to use powerful or piercing attacks in order to damage them at all.
A Bill Blaster appears in its Super Mario Bros. 8-bit sprite form in the background of the Mushroom Kingdom stage.
Bill Blasters appear as enemies in Smash Run in Super Smash Bros. for Nintendo 3DS. They're immobile, and attack by shooting out other enemies: usually, Bullet Bills, but rarely Koopa Troopas and Shy Guys - on rare occasion it may spawn a Souflee. Unlike most of their source games they can be defeated, and give extra items if destroyed before they run out of ammo.
Usually a big cannon with a skull on it is a clear sign that it's time to flee. Running away isn't an option in the Super Mario series, though. It may be known as the Bill Blaster, but it can occasionally fire Koopas or Shy Guys instead. Take it out before it's out of ammo for more goodies!
Usually, seeing a cannon with a skull on it would be your sign to run, and that's true in the Mario series as well...but with Bullet Bills speeding towards you, it's harder than it looks! Despite the name, they'll sometimes fire Koopas or Shy Guys in this game, just for variety. Take it down before it's out of ammo to get more goodies!