A tool like "object snap", but for snapping any point on a line (wall), to be more flexible in drawing.
For example, you have a room like this:
______________ | | | | | | <-- line A |_____________|
and you like to do something like this:
______________ | line A --> | | |_____________ line B | | | |_____________|_____________|
To draw line B, it would be useful to snap line A at the point, where line B comes up against it.