Covered with a sheet of glass they make pretty good cold frames for hardening off plants.
Planted up they're an attractive container for salad vegetables.
Ask your friendly wine merchant for some. Majestic, the wine warehouse outfit, will give you boxes in return for a donation to charity.
Just remember to drill some holes for drainage and to save filling the whole box with soil (it weighs a lot) fill the bottom with broken up polystyrene - like the type that comes with computer packaging.