This isn't original as such, I learned it back when I was into origami as a child, but it took me ages to realize how handy they are. It's not at all a complex fold, so it should be ok even for people not generally into that sort of thing. I use them continually especially for a quick container to hold screws, bolts, small parts, etc when taking something apart and I'm somewhere where a more normal container isn't around but a piece of paper is.

Step 1: Start With a Piece of Paper

Doesn't have to be unused, in fact it helps a bit if it's printed with text for horizontal/vertical straight guides. In this guide I'll be using a half-letter sized promo flier for the movie Eragon someone sent me, now way past it's due date. Larger pieces make larger boxes and the shape has effects on the height of the sides, though anything rectangular (within reason) should work.

Step 2: Find the Center

Fold and unfold to make a crease at the center. Either direction will work, but realistically going for the middle of the longest side tends to be best unless you need a really long but narrow box. If you have good eyeballing skills you may be able to skip this and just aim for the center.

Step 3: Fold Both Sides In

Fold in both sides to the center crease you made earlier.

Step 4: Fold in Corners

Fold in all corners, not all the way to the center but a little ways leaving some space as per picture. The two spaces on each side of the middle need to be the same or at least roughly the same (there is some leeway here). Here the text comes in handy for keeping the top/side of the folds straight in relation to the paper and as a loose guide to distance. Leaving larger spaces makes the box shallower but sturdier and vice versa, deeper but flimsier for smaller ones. Don't leave more then half of the distance between middle and side or you'll be in trouble in a few steps.

Step 5: Fold Out the Center Parts

Fold the space left in between the center and corner fold out toward the edges.

Step 6: Open It Up

Pull the two center parts apart. The sides will buckle inwards until it looks somewhat like a box.

Step 7: Crease the Edges and Corners

Pinch the edges and corners a bit to give them 90 degree folds.

Step 8: Done

Flip it over and put your stuff in. It isn't exactly the most sturdy container ever, but surprisingly so for being made of paper. As mentioned in the intro, it makes a great temporary storage for all those little screws and parts when taking something apart and keeps them in one spot that isn't the floor where they otherwise tend to end up.

It looks like a lot of steps, but really it's quite simple - I'm being overly demonstrative just to make it easier to follow for those without any origami experience.