1. Have a look at the top 100 keywords on 7search.  This will give you a quick idea of keywords that people are willing to pay big money for.  You can also type phrases into the 7Search Keyword Suggestion Tool.  This is just step one of your keyword research. You’ll want to dig deeper.

2. At FindWhat pay-per-click search engine you can do a search for any phrase and quickly see how much advertisers are paying per click. Amount shown at the bottom of each result.

3. If you have a Google AdWords account, you can experiment.  Pretend you are planning to advertise using different keywords, and see how much you would have to pay. That will give you a good indication of the popularity of the keywords.

4. You can also experiment with the Yahoo Bid Tool.  Let’s say you type in "asbestos cancer". 
The top three advertisers often pay about $10 per click.  So that would be an good choice for a topic – provided you’re a specialist on mesothelioma.

For "debt consolidation", the top two advertisers often pay more than $7 per click.

Also see http://www.nichebot.com