“I have done no passably decent job in this world which did not at first seem to me useless - absurdly useless, useless to the point of nausea. My secret demon is called :What's the use?”

The Notebooks of Leonardo da Vinci (1883), I Prolegomena and General Introduction to the Book on Painting

On His Blindness (1652)

Ai Weiwei, interview in “ Change http://www.pbs.org/art21/watch-now/episode-change,” Episode 1, Season Six, Art: 21—Art in the Twenty-First Century, PBS, April 2012.
2010-, 2012

Hagakure (c. 1716)

Half-Truths and One-And-A-Half Truths (1976)

Variant: Absorb what is useful, discard what is useless and add what is specifically your own
Source: Bruce Lee — Wisdom for the Way

Deathbed statement (November 1858), in response to a church minister who asked if he regretted wasting his life on fruitless projects; as quoted in Harold Hill : A People's History http://www.haroldhill.org/section_two/section_two_page_one.htm (2004).

Adams as portrayed in the musical 1776 (1969); this has sometimes been cited as an actual quote of Adams.
Misattributed

Part I : Ambiguity and Freedom
The Ethics of Ambiguity (1947)
Context: The failure described in Being and Nothingness is definitive, but it is also ambiguous. Man, Sartre tells us, is “a being who makes himself a lack of being in order that there might be being.” That means, first of all, that his passion is not inflicted upon him from without. He chooses it. It is his very being and, as such, does not imply the idea of unhappiness. If this choice is considered as useless, it is because there exists no absolute value before the passion of man, outside of it, in relation to which one might distinguish the useless from the useful. The word “useful” has not yet received a meaning on the level of description where Being and Nothingness is situated. It can be defined only in the human world established by man’s projects and the ends he sets up. In the original helplessness from which man surges up, nothing is useful, nothing is useless. It must therefore be understood that the passion to which man has acquiesced finds no external justification. No outside appeal, no objective necessity permits of its being called useful. It has no reason to will itself. But this does not mean that it can not justify itself, that it can not give itself reasons for being that it does not have. And indeed Sartre tells us that man makes himself this lack of being in order that there might be being. The term in order that clearly indicates an intentionality. It is not in vain that man nullifies being. Thanks to him, being is disclosed and he desires this disclosure. There is an original type of attachment to being which is not the relationship “wanting to be” but rather “wanting to disclose being.” Now, here there is not failure, but rather success.