I would not recommend editing files while the game is open. Please back up your save files before you edit them to avoid the risk of (permanent) save corruption. It's windows-only, but it should support both steam cloud and offline saves. It's useful if you want to immediately have everything available or if you're one of the people who got secrets but not achievements due to mods.
It currently supports editing the known stored values like mom kills/eden tokens/etc, collected items, and secrets.
Over the past two days I made a basic save editor for Rebirth.