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!
If anyway is bored I would love to see a graph of every possible HTML tag (old and new) plotted by frequency of actual use in the wild.
Some very rare - non outdated - ones:
How many of these do you know? And how many have you ever used?