Scary noise when standing and accelerating.
 

When I stand up to accelerate from a dead stop in a low (hard) gear I'll hear an awful clacking/cracking noise from below but with no way to determine where it's coming from exactly. I'm afraid something's going to break. Any ideas of what to check?

if you have a full suspension bike then that sound might be caused by a worn suspension pivot bearing. or the sound could be coming from your bottom bracket.

Sounds like BB needs lubed up, could also be chainring bolts or pedals.

BB disintegrated, get it serviced. someone has to look inside

Most likely the BB but I have also heard similar noises out of failing pedals.

None of the above. :D
it sounds like a rim flex same thing happened to me while I was trying couple of different rims so check your spokes and adjust them if necessary. :)

When I get this noise it's usually coming from my knees.

Failing that, grease the spindle-axle interface on a hollowtechII crank, if you have one.