What a wonderful question! Of course, since I write about HTML and teach intro HTML classes, I am likely much more familiar with the definition list <dl> than most people at the close of 2004, but let me see if I can list some of my favorite obscure tags anyway...
Here's my secret: rather than obscure tags, I'm actually much more fond of character entities. For example, I build my own bullet lists by using the • character entity, and then I can tightly control exact positioning by using my other favorite character entity, , a non-breaking space. One example of this is the newly revamped APparenting.com site: all the little bullet lists on the right are created with tables and • sequences.
And before you CSS purists complain, I want to point out that by using the • entity, I can wrap it in a font-size and thereby shrink it down or enlarge it to be exactly what I want. There's a lot of control you gain from CSS unordered list attributes, but changing the size of the bullet isn't one of them.
In addition, I'm addicted to the style attribute since it easily lets me tweak individual HTML elements to be "just so". A very common one: I use the <pre> tag for code listings on this weblog and if you look at the earlier entries, you'll find that they're all of the form <pre style="font-size:110%"> because without that tweak, monospace type on a typical Web page ends up noticeably smaller (read "less legible") than proportional type. Somewhere along the way I finally just defined a class called code, so more modern entries use <pre class="code"> instead. Much nicer!
I also still sometimes pull an <hr> tag out of my pocket for a quick horizontal line. In particular, I like being able to turn off shading, so a nice sequence is (using XHTML): <hr noshade="noshade" size="1" />.
One CSS attribute that I wish existed was font-color or text-color. It doesn't: you just use color instead, but if it did exist, it'd sure save me a bunch of recurring typos in my style sheets! And since I'm complaining about CSS implementation, I have to say that I've never seen a particularly fantastic typeface result from font-family: fantasy. But then again, how many people have fantasies about font families anyway? :-)
Finally, I like the CSS text-transform: uppercase within header tags as a nice way to change the appearance of headers on your page, and float:left or float:right lets you perform amazing tricks on containers that would otherwise be trapped in your layout.
How about everyone else? What's your favorite obscure or unsung HTML tag or CSS attribute?
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...and as to the usage - if you want all pre to be 110% of standard size define it that way in the CSS then all your blocks will be 110% with 1 line of code, loaded once by the browser. You save typing & we save bandwith.