if you are researching your Adsense niche and doing the standard keyword research here is another tool for you to use. www.seodigger.com will analyze any domain you enter and tell you what keywords it ranks for, there position in Google and the wordtracker searches.
This works great if you see certain sites popping up for the keywords you like, chances are they are also targeting low hanging keywords and you can see the ones they optimized for, kinda of like like letting them do the work for you!
Best of all its FREE to use and the free version does a great job. So combine this tool with the Google Keyword Research Tool and Free Wordtracker and you should be able to jump ahead of the competition