Found this list to find your niche keywords, using almost entirely Google tools. Nice compilation.
If you follow this, you are going to wind up with a massive list (or several). Narrow this down by editing the list to remove keywords having nothing to do with the main product you are developing. Take only those keywords which produce actual traffic monthly (Adwords has its accuracy problems, but you don't need to write articles for keywords which won't produce clicks.)
Again, check results in checkrankings.com to see what actual competition you have - and take only those which have high traffic and low competition. Also check these in Massive Keyword List Builder's Competition Finder. Once you scrape and peel down to the bottom line, you then should cross-check (MKLB does this more easily than going back into Google Adwords) for actual search volume.
It's a constant game of going wide and then narrowing down. You want to get the high demand, low supply keywords and then craft your mini-webs (and articles) around these.
"1. Build your keyword list using Google Adwords keyword tool to get search engine proven synonymic terms for the broadest term in your vertical market.
2. Then do a search on Google for the top 10 websites under those terms and then use the Google Adwords "site-related keywords" tool to pull keywords from the top 10 sites for each term.
3. Do a synonym search on Google Adwords for each of those terms from step 2 and add those to your keyword list.
4. Then repeat step 2 for the keywords you generate from step 3.
5. Then use Word Tracker, Keyword Discovery, etc. to find more keywords for each term you produced from steps 1 - 4.
6. Repeat the steps with Google Adwords for terms you produce from step 5.
9. Copy and paste your keyword list into the Google Adwords Traffic Estimator tool. Log into your Adwords account and click "tools" then click "Traffic Estimator".
10. Order the results from the Traffic Estimator from least traffic to most. This will be the order in which you complete your articles. Start with the least traffic ond work your way up to the top."