[chrishepner]

I use fieldset and legend frequently. I believe Bootstrap uses these in their example forms, so I don't think these are nearly as obscure as the others.

optgroup is occasionally useful for demarcating groups within lists, but this need hasn't come up much in my experience.

I also use figure and hgroup, although more just for the sake of using HTML5 elements than anything else!

[markdown]

hgroup has been dropped from the spec. You can/should stop using it :)