Peach Bomber is Princess Peach's Smash B move. She lunges her rear end towards her opponents and blasts them with a powerful explosion.
Using the Peach Bomber on most walls in the game will cause Peach to rise upward a tad. This can help with recovery on stages with vertical walls, such as Yoshi's Story and Fourside. The technique can also be used to stall a match, as Peach can continue to bomb the walls, keeping her alive and out of range for most other characters who would be KO'd if they were to try to go and attack her. Use of the technique in this way is banned in most tournaments.
In SSBB it has been confirmed in the Nintendo Conference Fall 2007 movie, though it has been altered visually to have a heart shaped explosion when the attack connects.