Many, many people have quotations in the signatures, so, shall we share our favorite "on topic" quotes with each other?
I'll get the ball rolling with a quote from one of my all time favorite authors.

I'll be more enthusiastic about encouraging thinking outside the box when
there's evidence of any thinking going on inside it.
-- (Terry Pratchett, alt.fan.pratchett)

"Between two evils, I always pick the one I never tried before".
Mae West

"How can I believe in God when just last week I got my tongue caught in the roller of an electric typewriter"?
Woody Allen

One of my favorites quotes, I use often with clients and pushy sales people.

“Don’t make me angry, you wouldn’t like me when I’m angry.” Dr. Bruce Banner/The Hulk

"Why am I calling you by your first names? I don't even know you. I still call my boss "Mister", and I've been working for him for seven years, but all of a sudden I walk in here and I'm calling you Rick and Sheila like we're in some kind of AA meeting... I don't want to be your buddy, Rick. I just want some breakfast". Bill Foster/Falling Down

"You see this? This is My BOOMSTICK!" Ash/Army Of Darkness

"Life is like a box of chocolates. A cheap, thoughtless, perfunctory gift that nobody ever asks for. Unreturnable because all you get back is another box of chocolates. So you're stuck with this undefinable whipped mint crap that you mindlessly wolf down when there's nothing else left to eat. Sure, once in a while there's a Peanut Butter Cup or an English Toffee. But they're gone too fast and the taste is... fleeting. So you end up with nothing but broken bits filled with hardened jelly and teeth-shattering nuts. And if you're desperate enough to eat those, all you've got left is an empty box... filled with useless brown paper wrappers."

~ William B. Davis (Cigarette-Smoking Man)
waxing cynical in The X-Files
["Musings of a Cigarette Smoking Man," Season 4]

"Some motherfuckers always try to ice skate uphill." - Blade, the first movie.

"Mr. Crowley, won't you ride my white horse, oh Mr. Crowley, it's symbolic of course."

Ozzy Osbourne

It's hard to pick any one Einstein quote, but if I had to it would be this:

"Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe."
--Albert Einstein

"Icing happen when the puck come down, bang you know, before the other guys. Nobody there, you know. My arm go comes out then the game stop, then start up."

-Denis Lemieux, Slap Shot.

"Remember, the enemy's gate is down." -Ender Wiggin to his troops before battle, from Ender's Game by Orson Scott Card

-Chess

"When in doubt, fuck" - Al Pacino's character, Lt. Col. Frank Slade, in Scent of a Woman.

And if I'm not mistaken, the censored television version is "When in doubt, make love."

Or,

"Legs. I don't care if they're Greek columns, or second-hand Steinways, but what's between them...passport to heaven." - Also Lt. Col. Frank Slade.

Not really 'on topic' but,
Spike Milligan's gravestone:

'I told you I was ill...'

I like Spike Milligan, big fan of The Goon Show

I always thought this was rather beautiful:

"There are not many persons who know what wonders are opened to them in the stories and visions of their youth; for when as children we learn and dream, we think but half-formed thoughts, and when as men we try to remember, we are dulled and prosaic with the poison of life. But some of us awake in the night with strange phantasms of enchanted hills and gardens, of fountains that sing in the sun, of golden cliffs overhanging murmuring seas, of plains that stretch down to sleeping cities of bronze and stone, and of shadowy companies of heroes that ride caparisoned white horses along the edges of thick forests; and then we know that we have looked back through the ivory gates into that world of wonder which was ours before we were wise and unhappy."
H.P. Lovecraft
"Celephaïs"

Lovecraft, another fantastic writer