It was her 3rd husband, John Lennon, who said Yoko Ono was the most famous unknown artist in the world.

I knew she'd been connected with the Fluxus movement in the 60s. Or thought I knew that, but apparently she didn't want to promote Fluxus, she wanted to be independent.
She got involved in the art scene while at Sarah Lawrence college, after a childhood in Japan (and other places - her father was a banker and moved around). She was born in 1933 and was in college in the 50s (she started by briefly studying philosophy in Japan's elite university) and afterwards got a live-work studio loft in New York, which was used for concerts (John Cage was part of her crowd) and performances.
Her best-known performance is "Cut Piece" - where members of the audience came and cut away the garments she was wearing, leaving her naked on stage. She's performed that many times in many places, with different audience reactions. You can see a snippet of the 2003 version
here.
Her 1964 book "Grapefruit" - with instructions for things that are to happen in the reader/participator's mind - has been reissued in the 70s and again in 2001.
She's had an extensive musical career - I like the animated music video "Walking on Thin Ice" which you can see
here. This was the song she and Lennon were working on just before he was shot. Earlier this month she was in London to perform with Ornette Coleman, and was
interviewed on the the Today programme along with her son, Sean (b.1975).
She made films between 1964 and 1972 - "No.4" is also called "Bottoms" and consists of a view of buttocks while the performer walks on a treadmill, with soundtrack of interviews of those being filmed, as well as those considering joining the project.
And she's an activist for peace and human rights.
In 2007 she unveiled the
Imagine Peace Tower in Rekjavik, Iceland, a tower of light that is visible between 9 October and 8 December each year.