Although the name might not be familiar, many people have seen a Pickelhaube without realising it, for it was the standard spiked helmet of the German army during the first two years of
the Great War. The name comes from the German words 'pickel', a transformation of 'becken', or 'bowl' - and not 'pike' as one might expect - and from the word 'haube', which means 'hood'.
The helmet had been introduced in 1842, and continued in service until 1916. It was actually made out of leather, polished to a deep shine, with a spike on the top and a large eagle crest on the front, although the helmet was usually covered in cloth as an aid to camoflague. The brutal clarity of World War I put paid to the pickelhaube, and the 1915 model - on which the spike was removeable - was the last. In 1916 a new, all-steel helmet was introduced, a helmet which is also familiar to modern eyes, as modified versions continued in service until the end of the second world war. Indeed, the modern US Army helmet derives from Germany's pickelhaube replacement.
It's worth noting that, right up until the end of the 19th Century, European armies tended to dress in bright, colourful clothes, topped off with impractical felt hats, all of which were liabilities in an age of machine-guns, heavy artillery, and ever-present shrapnel. By 1914 all but the Austrian army was dressed in dull shades of green and grey - the Austrians persisted with dress uniform but were shocked into action by a series of defeats at the hands of Russia and Serbia.
The pickelhaube was mocked outside Germany at the time for looking slightly ridiculous, although the spike was not designed as an offensive weapon, and German infantrymen were not expected to charge the enemy head-first. The spike was actually for cosmetic reasons - to look fierce and impressive - and had a secondary role as an aid to identification. The practice of designing military equipment on the exclusive basis that it should look good died a death in the 20th Century, but in an age of 'perfect discipline' and robotic drill, where willpower and character were regarded as more important than machine-guns and howitzers, the thought of soldiers drawing reserves of strength from their uniforms made a certain kind of sense.
After 1916 the steel helmet reigned, and reigns still, although many armies also use kevlar. Whilst nothing short of centimetre-thick steel plate could protect a soldier from a direct bullet strike, the vast majority of battlefield projectiles were in the form of small chunks of metal broken from the exploding cases of artillery shells. Shrapnel was unpredictable and varied in lethality, but the smaller fragments could easily be stopped by a metal hat, and thus modern infantry headgear came into being. Since then, helmets have had many uses, as cooking pots, seats, and handy places to write slogans such as 'meat is murder' or 'bury me upside down so the world can kiss my ass'.
Many thanks go to liveforever for clarifying the translation (I had originally written that 'pickelhaube' meant 'pike helmet', which sounds plausible but is wrong).
Selected sources
http://www.worldwar1.com/sfgph.htm
http://www.kaisersbunker.com/pe/pe.htm