Casual & Friendly:
* What's good?
* What's crackin'?
* What's up, doc? (A playful reference to Bugs Bunny)
* Sup?
* How's it hangin'?
* How's it going?
* Yo!
* Hey there!
* Hey man/woman!
Informal & Cool:
* Whaddup?
* What's the deal?
* How's it goin'?
* Been busy?
* Long time, no see!
More Informal & Playful:
* What's the word?
* What's poppin'?
* What's new?
* How's life treating ya?
More Specific:
* How's your day going?
* What's new with you?
* Whatcha been up to?
Remember: The best slang to use depends on who you're talking to and the situation.
It's always best to use slang that you're comfortable with and that you know is appropriate for the context.