The two best known octets in classical music are probably those by Felix Mendelssohn (which is for a double string quartet) and Franz Schubert (which is for clarinet, bassoon, horn, violin, viola, cello, and double bass). Igor Stravinsky also wrote an octet for wind instruments (an unusual grouping of flute, clarinet, two bassoons, two trumpets, and two trombones) and Paul Hindemith wrote a lesser known piece for clarinet, bassoon, horn, violin, two violas, cello, and double bass. Dimitri Shostakovich also wrote a string octet which utilizes his typical atonal techniques difficult for musicians.
"Octet" is also rarely (and strangely) used as a name for a vocal performing group, not necessarily of eight members, as proven by the Glens Falls High School Octet in New York State. This particular group has forty-nine (49) members.
Saxophonist David Murray leads a famous experimental jazz octet, the David Murray Octet.