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 respawn a short time after dying at the exact location where the puppy died. If this happens, simply walk up to the now-resurrected puppy and tell him to follow you. This works even if you looted edible dog meat from the puppy when he was dead! (confirmed Xbox 360)

After the puppy spawns at Vault 101, it may become stuck permanently in the ground (confirmed Xbox 360). It will act and move as if it is fine, but upon interacting with it, the puppy will behave like a corpse, allowing you to loot it, but it is carrying no items. This makes speaking with the puppy impossible. If this happens, attempt to reload your file from any previous save and retrieve the puppy again. This may not work, however, resulting in the puppy becoming useless.