LibreCAD

A simple to use 2D CAD program. Often a heck of a lot easier to draw gridded outlines in than GIMP.

Left click to select, right click to cancel the tool you have selected, 'Esc' twice to deselect all.
Draw a selection box from left to right to select anything inside the box
Draw from right to left to select anything intersecting the box
To move a vertex, click the object to highlight it, then click and drag over the vertex (in blue)
To quick-move something, highlight all the parts you want to move, then click and drag on a line
To use move tools, select the object, go to 'Modify' -> 'Move / Copy' for example, hit return to choose what you've already got selected, then choose a reference point and destination
To move something to another layer, activate the target layer, move the object selecting 'Use current layer' and 'Delete original'

Tutorials