Did Dogmeat die at a Super Mutant camp? Use the editor to change his "RefID" status from Dead to Alive. You must be in a different cell (e.g., inside Megaton) to avoid an instant re-death.
If you started Fallout 3 before buying Operation: Anchorage or Broken Steel, your save is unmarked. Use the editor to add the DLC quest starter items (e.g., "Outcast Radio Signal" holotape) to your inventory.
Nothing ruins a game faster than infinite health and infinite ammo. The struggle of surviving the Metro tunnels or scraping together 2,000 caps for a unique weapon is the soul of Fallout 3. A save editor is best used as a scalpel, not a sledgehammer.
Recommended "Fair Use" Edits:
Follow this exact workflow to avoid corrupting your 100-hour playthrough.