Great question and terrific advice and resources given already.

What I did a couple of years ago was a mashup between and article directory
and a blog. I took a niche topic and added a ton of free content then blogged about that content.
It has become my highest traffic and best converting site. (Click to see the article directory/blog mashup)

Since its launch, I've been using Entrecard to find ideas for niche sites and
new approaches for marketing to those niches.
I just login and check the new blogs in each category, then I'll check for
the most popular blogs and the most recommended blogs in each category.
The good ones go in my contact file for link exchange and joint venture possibilities.

Best of Success,
Stephan