If you're a sci-fi writer and you're strapped for ideas (which probably isn't likely), or if you just like to think about all sorts of different stuff, you could do worse than to check out The Edge's question center.

It's a bunch of really smart people asking questions, then answering them.

Sort of.

Some samples:

"Is our brain smart enough to understand the brain?"

"Are space and time fundamental concepts or are they approximations to other, more subtle, ideas that still await our discovery?"

"Why do people like music?"

"What is the missing ingredient — not genes, not upbringing — that shapes the mind?"

"Is God nothing more than a sufficiently advanced extra-terrestrial intelligence?"

"Is the universe a quantum computer?"

"How different could life have been?"

And so on...

There's lots more. Check it out.