Canned response: The crime rate, taxes and my blood pressure.
Depends on context:
Someone I see everyday: What’s up?
Tremorviolet: Nuthin’ much. What’s up wit’ you?
Someone I hadn’t seen in a while: What’s up?
Tremorviolet: Duuuuuude! Man, it’s been forever. How’s it goin’?
Yes, it’s incredibly lame but I really do talk like that…
Unless I have strong feelings about how I’m feeling I say, “not much, howeryou?” Otherwise I say “I’m really grumpy today”* or “great! Howeryou?”
*I like to acknowledge that I know I am grumpy so if I happen to slip folks will take it with a grain of salt.
My possible responses:
- Hey.
- S’going on?
- Not too much.
I think my brain has some sort of random algorithm designed to pick out a response. It’s just about guaranteed to be one of those three, though.
“Up” is the direction in which the local gravitational field strength decreases most rapidly.
She really does say this. Sometimes multple times in one evening in response to my husband asking her, “What’s up?”
Acceptible answers:
“Not much”
“Nothin”
“Hey”
“Your mother”
“My cock”
You are correct, it is the only proper response. Some variations:
“Hey, whassup man” followed by a soul shake and maybe a soul shake/back pat if y’all are real cool.
" 'sup " accompanied by a short, upward headnod.
I always need a few seconds to remember that it doesn’t mean jack. Once I do remember it, I answer with something else that’s also meaningless, like “hi there” or “good morning”.
Back home, if people ask something like “what’s up?” or “what’s going on?” it’s an actual question and they get an update on family affairs (which is also what I get if I ask them).
I usually say something along the lines of “Not much, what are you up to?”
Sometimes I’ll get a little wacky and respond with “Oh, nothing good”. But I’m just crazy like that.
Back in Junior High/High School years I would occasionally answer,
“Up is a relative concept having no intrinsic value.”
But eventually I got tired of being a smart-ass so I haven’t used it in a decade or so. Things days I usually answer with a “not much” or something similiar unless of course there actually is something “up.”
The big expection is if I’m passing someone in the hall or other similiar situations where there really isn’t much time for a a reply. Those seem to frequently involved “what’s up?” being answered immediately with a second “what’s up” as a reply and then both parties walk on happily with neither one of their queries being answered.
I’ve actually given this some thought, because “what’s up” is a very common greeting among my peer group, mostly 20-somethings. My theory is that how you respond to this is actually an indicator of your hipness.
It seems to me that the hip, cool, happening (mostly younger) folks respond to “what’s up?” with another “what’s up” or, as another poster mentioned, just “sup.” It’s a statement, really, not a question. Like “hi” or something. So you get two people having an entire conversation as follows:
“sup?”
“sup.”
(with little head nods)
I’ve observed my younger brother do this, and it’s quite odd to me.
The less hip, happening, cool (like me) actually treat it as a question and respond “not much” or some such. So, sorry to all who say “not much,” but this is an instant indicator that you’re over the hill. 
How about people like me who actually tell you what’s new?
Gosh, you’re a positive relic! 
I usually answer “Nada. You?” because face it, nobody really gives a damn. But I do like the answer of actually telling them and watching them trying to get away quickly, because what they really want to hear is “nothing” 
However, the best answer I ever “heard” (in an AOL chat room of all places):
“Up is a two letter word meaning above.”
Sometimes, “I was hoping you knew.”
Ususally I just say Hi.
“Everything!” or “Ever’thang.” Ever’thang is ever’thang. Yeah.
“It’s the direction of negative gravitational gradient, typically normal to a planetary surface. But that’s not important right now.”