My locker at the local hackerspace was a mess. I made a shelf to fix that. It cost me nothing but 10-15 minutes of my time. Here's what I did.

Time: Barely any
Skill: Not much
Cost: None

Step 1: Materials

Required

Corrugated cardboard; the thicker the better
Knife

Optional, But Will Make Things Easier

Tape measure
Pencil
Ruler or other long, straight object

Step 2: Building It

I work in a music store. Tubas get shipped in boxes made of this wonderfully sturdy cardboard. I've had several sheets of it kicking around for a while now, and decided this was a good use for them. Home appliance boxes (fridges, washers, etc.) are also good. Raid the trash behind an appliance store, or steal the box from a neighbour's new stove. Alternately, layer several thinner sheets for strength if none of the thick stuff is available.

I trust everybody has something knife-y already.

Measure the the locker/cabinet/wherever the shelf is going. Either use the tape measure or just hold the cardboard up to the thing and mark off the size needed. A few millimetres off won't make too much difference, but it's better to have a bit too much than not enough.

Once the top part is cut, measure the desired clearance and cut the side supports.

Run the corrugation "grain" crosswise for the shelf platter, and vertically on the supports, for extra strength. Thinner stuff especially can begin to sag under heavy loads.

Step 3: Organize All That Crap

Get that mess out of there, toss all the useless detritus that has accumulated since the last time it was emptied, maybe take the time to really scrub it out, and then get the shelf in place. Put everything back in and enjoy the new second storey of storage space. If there's enough room, why not even add another shelf on top? Or several shelves, if it's a really tall locker.

Want it all fancy? It's cardboard. Once it's sized to fit the locker, grab some craft glue and old magazines and découpage it. Paint it. Papier-mâché it.

The plastic boxes stuck to the door are my Sugru'd Dremel burr cases.