I know that many people dislike very distinct (geometric) bokeh shapes and view them as a gimmick which is fun for a second and then more annoying than anything else. While I agree that bokeh like that can distract from the main subject and narrow the possibilities you have with a particular lens in certain instances, I think it can be used in creative and fun ways, particularly with adapted lenses, where you have a lot of opportunities to consider the options in your adaption process (like variation of FFD, rotation of the lens, using a lens normal or reversed, use tilt/shift adapters etc.)
So I'm very curious about some of the lenses you know which feature interesting and unusual bokeh shapes, and if you have tried them perhaps also seeing a couple of sample shots made with them.
EDIT: There has been a very interesting thread here which goes in depth on background blur and bokeh. As part of the interesting discussion there, @TomAxford (who has already shown some interesting triangle examples below) has provided an explanation and some nice looking examples of "Shaped bokeh", where you create your own bokeh shapes, and let your fantasy go wild. While that approach is different to using adapted lenses with certain bokeh shapes built into the lens, it is an interesting and significantly more versatile approach to getting creative with bokeh.
I highly recommend checking it out here:
dprevived.com/t/bokeh-how-much-background-blur-will-i-get/2398/post/38073/
and giving it a try yourself! Thanks to @TomAxford and to @TimoK and @nzmacro for recognizing the overlap!
I'll start with a simple, very cheap enlarging lens in two focal lengths with square bokeh:
Meopta Belar 50 & 75 mm f/4.5
Usually that's how well I fit in... by simple.joy, on Flickr
I‘m completely crazy… but a square shooter! by simple.joy, on Flickr
(used rotated)
Don't call my heart 'low res'! by simple.joy, on Flickr
(used slightly tilted)
Because the Belar lenses are slow you have to get quite close in order for them to work properly, or the background bokeh far enough away but keeping that in mind, I'm sure they can be used to great effect, even outside of macro stuff. I'm curious if there are some faster (taking) lenses which feature very square bokeh like that... If you are aware of some, I'd appreciate it, if you can let me know.