In need of a free in-game level editor!

That will depend very heavily on your game. I know there are some assets on the store which allow you to do a certain amount (sorry, can't remember their names). However, if you need specific functionality for your game (and you probably do), then you'll need to write your own.

It isn't actually that difficult to do, just time consuming and you'll need to really thing about the structure of your game and your code architecture. Lot's of interfaces for things like IPlacableObject will help you out here.

EDIT: Just realised you were asking for a free solution. I think that given how specific in-game editors are to the games they are made for, you'll be very lucky. However, if you're looking for more of a modding solution then you might want to look at how games like Kerbal and Cities Skylines achieved this.