Puppies!

 is a perk added to Fallout 3 with the Broken Steel DLC.

With this perk Dogmeat is reincarnated as a puppy every time he dies and can be added back to your group when the "puppy" goes to Vault 101. Dogmeat's puppy seems to be much tougher than the original Dogmeat.

Despite the name, this perk only allows you to get one Dogmeat's puppy at a time. If it dies, it will be replaced by a new puppy.

This perk still works if Dogmeat died pre-Broken Steel.

Glitches
You may have to wait weeks at a time to get a new puppy, or sometimes a new puppy may never come. This is the case of the puppy dying in strange areas, or not loading in previous areas.

Sometimes, the puppy will be resurrected after dieing. If this happens simply walk up to the puppy and act as if you left there.