Matali Crasset has officially made it into my book of the most out-there designers with the most-difficult-to-navigate websites, but peeping her user-configurable seating is well worth enduring the torturous interface. Since the '90s, the experimental designer has conceived of her work as "a research movement, made of hypotheses more than principles."
We'll start off simple. Her 1997 Il Capriccio di Ugo armchair featured armrests that could be flipped down into trays:
Crasset's Decompression Chair from 2000 featured a "backpack" of sorts made from a parachute-like material, and could be inflated to turn the entire thing into an armchair:
Her Digitspace seating from 2003, designed for public spaces, consists of one simple, repeatable shape that lends itself to a variety of configurations:
More recently her Dynamic Life sofa, done last year for Campeggi, goes from sofa to lounge to daybed, depending on which sections the user chooses to flip outwards.
Crasset designs a lot more than furniture; dive into her site (hopefully you'll have an easier time of it than I did) to investigate her other projects here.