Hello,

I was wondering...is there an easy way to make lines that are grid-snapped and straight look hand-drawn? Some kind of filter effect maybe...or a simple technique?

Use the assistants for that.

Wow! The Assistant tool is AMAZING! However, I need to take a existing drawing that has lot's of straight lines and make those lines look more hand-drawn. Is there anything I can do to achieve that?

Is it the fact that the lines are straight that's the problem or that they're 'clean' with sharp outlines and an even colour?
What do you mean by 'hand-drawn'?

The appearance of the lines themselves can be changed to make them more uneven in outline and textured using a variety of methods.
Making them not-straight is more difficult and depends on whether you want them to start and end in the same place as before.

As a quick look-see, you could try the GMIC filters in the Deformations and Degradations groups.
It also depends on if the lines are on a transparent layer or if they're on a coloured/white layer and if there's anything else on the same layer.
For a small number of lines, it may be easier to redo the drawing and make them really hand drawn.