Ah yes, we've had discussions about these on the chatz many times. I see "thus" and "ought" relatively often, I think, but "lest" and "shan't" sure are rare. I have a whole list of words that have lost almost all their use, but should come back, like...
whence, thence, hence (in it's locative use), whither, hither, thither, wherefore, herefor(e), all of these + -soever, heretofore, nary, eke [cause it sounds so cool (and is one syllable shorter than "also"... >_>)], ...
Also, maybe we should start using the word "irregardless" with the sense "with regards to". It could, irregarless of the possible amount of our use of it, change its meaning to what it /should/ mean. \o/