Introduced in 1984 by Tony Magelica's Mag Instruments company, this light has been hailed (by Fortune & Money magazines, plus American Style, a book depicting classic American product designs) as an example of outstanding engineering design. You've likely seen one, but if you've never used a Mini Maglite, you may be unaware of its many clever features:
All this in under 6 inches (15cm) of length, weighing under 4 ounces (114g). The Mini Maglite has many imitators, but nothing comes close to the original. And you can buy one, including two high quality alkaline batteries, spare bulb, and a carry holster, for under USD$10. It's hard to imagine a better incandescent travel flashlight; were it not for the advent of vastly superior LED designs, this would still be the perfect choice.
If you already have a Mini Maglite, and would like to convert it to an LED model without losing the focus capability, this can now be done with a conversion kit (most such kits aren't much of an improvement, but this one is a reasonable choice). If you're not a Mini owner, though, you'd be better advised to choose one of the models I discuss on the main page.