Map editor
This page is heavily a work in progress.
Factorio comes stock-standard with a map editor. You can access it by selecting "Map editor" on your Factorio main menu.
The editor is much like in-game, except there is no player, and you have unlimited resources.
Placing Entities
To place an entity (labs, powerlines, transport belts, mining drills, etc...), select "Entities" from your toolbox, select a tool, select what you want to place down, then click somewhere to place that item down.
You have three tools.
Cursor
Appropriate for most situations. Places one of the selected entity at the current cursors position. Much like the normal in-game mechanics.
Tip: Pressing "Q" on your keyboard will put the item you're holding away.
Spray
Good for resource piles and trees. You can select a size, intensity and speed, the larger the size, the more area that the spray places entities, the higher the intensity, the more often entities get placed, the higher the speed, the more rapidly the spray continues spraying while holding down the mouse button.
Paintbrush
The paintbrush is virtually the same as the spray, only difference is that if you hold down the mouse button, the tool won't continue to place entities. Also good for resource piles and trees. Whether you use spray or paintbrush is personal preference.
Deleting Entities
Cursor
When using the cursor tool, you can hover over the entity you wish to delete, and press "X" on your keyboard.
Spray/Paintbrush
When using either the spray or the paintbrush, right click to "unspray" - though, this process can be slow, using the cursor to delete entities is much more efficient.
Placing Items
You can place items for the player to pick up in the editor. To do this, select the "Items" tab. From there, select what you wish to place down, then hover your mouse to where you wish to place the item down and right-click.
To delete items that you've placed, just like when placing entities, press "X" on your keyboard while hovering over the item you want to delete.