One of the common issues when blogging for a living is finding potential niches. There are lots of best keywords finding tools, some of them free, some others with money. One of the free tools is the free Adsense keywords bid value query, developed by two SEO professionals, Glenn Scott and Dan Murdock, under the umbrella of Symbiotic.
Besides this tool, Symbiotics offers for free a list of all Google Adsense keywords sorted alphabetically and also a Top 1000 best paying Adsense keywords list. The most interesting point is that you can see not only the value per click, but also an estimation of the total revenue per day, which is an invaluable tool, as at the end of the day you want to know how much money can you earn with those keywords. Think what is best: to have a keyword paying $50, which is searched for twice a day, or to have a $5 keyword searched for 1000 times per day?
Later update: the Symbiotic tool is not working anymore, so I would suggest you start the research from Wordtracker, then check the bids values in Google AdWords. For that, you’ll need to open a Google AdWords account, which means you’ll need to make an investment of $5 (the one time fee required when opening the account).