I named 50, but I really should have remembered more, like H1 through H6 (duh), HR, SMALL, SELECT, OPTION, OPTGROUP, NOSCRIPT, and NOFRAMES. Oy vey. BDO and KBD were new ones to me.
Here are the ones I entered: A, ADDRESS, B, BLOCKQUOTE, BODY, BR, CAPTION, CENTER, CODE, DD, DIV, DL, DT, EM, FIELDSET, FORM, FRAME, FRAMESET, HEAD, HTML, I, IFRAME, IMG, INPUT, LABEL, LEGEND, LI, LINK, META, OBJECT, OL, P, PRE, Q, S, SCRIPT, SPAN, STRIKE, STRONG, STYLE, TABLE, TBODY, TD, TEXTAREA, TH, THEAD, TR, TT, U, UL
@funkdaddy, BASE used to be really common. It’s supported by every browser, AFAIK. It’s usually used to set the base href for a page.