Your mother always told you to wear fresh panties so that if you were hit by a bus you wouldn't be embarrassed by the soiling as you screamed in agony and paramedics worked to staunch the internal hemorrhaging. In the spirit of that eminently practical advice, I've decided to put together a few suggestions for bras which could be useful in an emergency.
An inflatable bra could save your life in a situation in which a minuscule amount of extra flotation is needed. Such situations include unexpected poolside dunkings, ballooning holidays, and of course, plane crashes. Inflatable bras could also earn you extra favor in a sticky situation by increasing your helpless female quotient by a factor directly proportional to your apparent cup size.
Of course, if you happen to be considering a career as an assassin, a bra which doubles as a gun holster might be useful. You can read more about my thoughts on the gun bra and its creator here. If you can't be bothered going there, then basically the gun bra is a bra with a holster which can hold a snub nosed pistol and a can of pepper spray. I'm guessing you wouldn't be wearing this bra under sheer clothing, but then again, if you subscribe to the theories of the designer you'll probably be wearing body armor at all times anyway.
Not only do water bras enhance the cleavage, but they can also be handy in the case of finding oneself in a desert, concert, or when trapped on the tarmac for hours at a time due to unforeseen airline strikes. Water bras can be 'hacked' to conceal almost any kind of liquid at all (unpleasant thought here, let's hope nobody tells the folks at the TSA, or it'll be a friendly mandatory chest grope by sullen looking lasses with guns on your way through security next time you're at the airport.
An armored bra has obvious advantages over the common cloth bras which allow for all sorts of tomfoolery. Armored bras are excellent for warding off casual gropers. German policewomen do actually wear special 'armored bras' designed to go under bullet proof vests. Whilst the bras have no actual armor in them (unlike the illustrated bra, which is made from kneepads by the talented fellow who runs MadMaxModels.com.), they do reduce the risk of injury to the women wearing them, which is nice.