The B'Omarr Monks are an ascetic religious order in the Star Wars universe combining elements of transhumanism and Gnosticism. Their first appearance was during a minor glimpse in Return of the Jedi when C-3PO enters Jabba's Palace. Despite this the expanded universe has gone on to establish a rich background for them. Not bad for a second of screen-time at the edge of the frame.
The B'Omarr monks originally moved to Tatooine at some time in the distant past, centuries before the timeline of the films. There they built a monastery on the edge of the dune sea that would eventually become Jabba's Palace.
They practiced a form of strict asceticism and spent nearly all of their time in meditation on the mysteries of the universe and what they referred to as the "ultimate truth". Eventually a monk will be determined by his fellows to have become "enlightened" and at that point the lesser monks will aid him in having his brain removed from his body. The body being viewed as merely a distraction. At this point the monk's brain is placed in a jar filled with a nutrient solution and is safely stored away where he can while away the millenia in contemplation.
Occasionally a monk will have need of slightly more mobility and his brain jar will be attached to a large, spider-like droid. From experience the Arakyd BT series perimeter droid is typically used, particularly the six-legged BT-16.
After Jabba the Hutt was killed the monks were largely able to reclaim their monastey. Jabba's major-domo, Bib Fortuna, briefly had his brain removed and placed into a BT-16 doid, but was later able to have it re-implanted into another T'wilek body.
As stated above the only appearance of a monk in the films is during a very brief sequence in Return of the Jedi where a monk is seen in a BT-16 droid. Apart from this they have made many minor appearances in other media.
One of the largest intial appearances of the monks was in the book Star Wars: Tales From Jabba's Palace edited by Kevin J. Anderson. The book largely concerns the time after Jabba's death when the monks are trying to regain control of their monastery and as a result they tend to play roles in most of the stories as minor characters. A second book, dedicated entirely to the monks was also released titled The Brain Spiders and written by John Whitman.
While relatively minor chracters in the Star Wars universe at least two action figures have been released of the monks as well as other incidental figures.
The first of these was part of the Power of the Force line released by Kenner. In 1997 it was made available exclusively via their website for approximately $10 (the offical Star Wars site states $10, a fan site states $7.98, I don't personally recall what I paid for mine). Despite being listed widely as an internet exclusive (and referred to as such on the Kenner website at the time) they were also apparently available in 1997 via a coupon received with the Spirit of Obi-Wan figure (itself a special available through a promotion with Frito-Lay) if sent in with the proofs-of-purchase of 3 other Star Wars figures.
The 3 ¾" figure was shipped loose and unassembled in a white cardboard box with an instruction sheet. Collectors will no doubt be interested to know that there was a slight variation made to later versions of the instruction sheet to add a choking warning due to the small parts. This version is unique among later releases in that it that the fluid surrounding the brain is colored blue. Later figures use the pink fluid seen in the film.
A later version of the figure was released in April 2004 as part of the Ultra series of Hasbro's Saga line of Star Wars figures. Here he was only sold as part of a three-pack titled "Jaba's Palace: Court Denizens" along with Bubo and Wol Cabasshite which retailed for, again, approximately $10. This version featured a slightly modified paint job with the brain jar filled with pink fluid and a slightly darker, more metallic look overall. It was also slightly less articulated than the previous release.
The monk figure showed up as well in January 2003, this time in Lego form as part of the Jabba's Palace set. Here he was an actual figure to assemble and not a minifig
A miniature was released in the Universe expansion to Wizards of the Coast's Star Wars Miniatures game on August 19, 2005. Here it was a rare figure, #15 out of a set of 60.
The B'Omarr monks seem to occupy a place in the Star Wars universe known all too well by many others: a character glimpsed for a brief moment in the film and since expanded upon heavily by the books and toys. Whenever there is a need to write a story that involves brain-swapping or produce a cool-looking, but minor figure the monks will be there.