I've only had the one low speed off in my current commuting/recreation career over the past two years. Going round a corner where I play badminton there's a tree which had started shedding it's leaves. A recent shower had left the ground wet and when I checked after the spill that corner felt like it was ice! No harm done though.
Other than that, apart from the usual childhood spills the only other off I've had was completely self-inflicted. I was riding back from the pub one night and at the time I had those old Ever-Ready lights mounted on the forks. I noticed the front light bracket had twisted so the light wasn't shining forward, but rather than stop I decided to give the light a kick to twist it back to the right angle. A couple of dunts had no effect however, so the next kick carried some force.
Unfortunately the kick only struck a glancing blow to it's intended target, and the force carried my foot through the spokes, which then wedged it against the forks, thus arresting the bikes forward motion. This resulted in me flying over the handlebars (happily dislodging the foot in the process), executing a 1/2 somersault and sliding to a halt on my back.
Luckily there was no other vehicle nearby, and I was wearing a thick jacket for the cold which seemed to take most of the force. So after dusting myself off and fixing the light bracket I was on my way again. I don't even remember being bruised, but that was probably the effects of the alcohol, bringing a ying-yang life balancing element to the story.
So in conclusion, my rate seems to be one every decade, and if I can work on the common sense I even have room to reduce it!