I use my AWStats page on my websites to figure out what to write next. Now, this only works for websites that are currently getting traffic - it doesn't have to be an amazing amount of traffic (the website I'm taking these examples from only gets ~1000 unique visitors per month) but you do need some traffic.
One of the things I never understood was how in the world people found my website with obscure keywords. I mean, I had targeted specific long tail keywords - which do show up in the search logs - but other things kept popping up.
It turns out that people are absolutely desperate for information. The average user has no idea how they can properly use Google to retrieve information, and instead enter in anything that pops into their heads; for example:
"computer build recommendations best machine for the money"
Of course I didn't get a lot of hits for this keyphrase - only 2 - but the keyphrase was enough for me to figure out what this person was looking for ... and what other people are looking for:
The best machine for the money. Or, the best computer they can buy on a budget.
Bingo, I've got an article ... all without doing hours of pain in the ass research.