The idea of a collapsible water bottle is not new, but I always enjoy seeing the different implementations. Rather than going the roll-up path like the Platypus or Vapur, the Aquatina simply collapses downwards into a tiny, circular package that would not look unsightly in one's jacket pocket.
The Aquatina was featured a few years' back on the BBC's The Dragon's Den and while the designer/manufacturer is based in the UK, the bottles can be purchased locally via Aquatina Australia, though I don't believe the site is associated officially.
In its extended state, the Aquatina can carry 500mL of liquid. Collapsed, it comes in at around one-third the size. For the environment and health-conscious, the plastic is BPA and PET free.
If you're keen, they're $15 a pop from Aquatina Australia (includes postage), though a search of eBay might turn up cheaper options.