My initial intention was just to make a shallow pond to attract frogs and the like, to keep the slug population under control. Then, I aquired a tractor tyre (don't ask), and thought it might be fun to make the pond out of that. As it happens, I was over zealous with the digging and the hole got too big, but I plonked the tyre in on top of the pond liner anyway, and it acts as a nice platform for the frogs to creep up on.
I've also got an old branch at the back (give the pic a click and it will be bigger and easier to see) which gives a chance of escape to any critters that fall in by mistake. I've put a load of oxygenating plants in the pond (mostly submerged) and these, along with the weed on the top which help exclude the sunlight, stop the water going green.