Here is an image I discovered while researching my post "Practical uses for Bokeh". It was done with an old, but effective technique.
The basic idea is the hand hold (or tripod mount) a very slow shutter speed exposure and "drift" during the exposure. This was 5 seconds at about f11 using the Olympus 25mm f1.8. I forgot to disengage the stabiliser. The image may look better or worse for it.
Here is one of the same subject matter (different angle) "normally" taken.
Variations on this are to extend the time so the image starts to become more coherent again, zoom the lens during exposure or take the bulk of the image normally, only messing with it at the end or beginning for a while.