A bag of 10mm (or 6 inch) nails, a hammer and a file or two (or a grindstone, or some rough concrete, or a rock) can be used to make some pretty useful things in a pinch. Without further ado, here are some ideas:

1) Leave the nail exactly as it is - useful for banging quick holes in things). Can also round the end with a file to make nice round holes.

2) Flatten the end, sharpen it to make a chisel. Useful for cutting thin metal (like a tin). Lots of info and diagrams at nzdl - chisel

3) A drill, also described at the nzdl link.

4) Flat and Phillips screwdrivers. Flat is simple - file it to a flat point! But Phillips? Well, ok, 2D phillips... but it still works! Basically make a flat triangle at the end...

5) Various other pokers and pointy things as the need arises - the whole point is that you make what you need. Filing away 5 or 6 sides and adding a right angle bend for example could make a crude allen key, etc.

Notes:

• All these tools can be fire hardened by heating up to red hot then quenching in water a few times.
• Obviously these don't work as well as a 'proper tool' in some situations, but they are cheap, easy, and disposable (so if it might break, rather break a $0.05 nail than a $5 screwdriver).
• Sharp tools are more fun - sharpen on a file or sand paper.

Here are the ones I have made that are lying within reach right now (I made them recently away from home):