I found this in a colleague’s office the other day. It’s the Ekstrem chair by Varier and is the epitome of cool ergonomics. Yours for £630, I believe. Surprisingly comfortable, too.
The same firm sells something called a ‘Gravity Recliner’, tastefully illustrated thus:
Available for a mere £1144.99.
On the other hand, you can hang upside down by your ankles for free.