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It has nothing to do with DMOZ. DMOZ probaly won't list a site like this unless you have a currupt editor.
My most successful site is, as a consequence, going to remain ad free, the income is simply not worth lowering the integrity of a resource site in my case.
Just my experience so far
is there a way to see how often a phrase is searched for on Google?
Not that I know of. You can look at the Zeitgeist to see the most popular searches, which are usually celebrities, disasters, movies etc.
In my limited experience, I have tried about 15 niches and figured out the ones that pay the most. This knowledge in itself is a starting point, but the time investment to write content (or find someone competent enough to do it) is a stumbling block. I have focussed, instead, on building more sites. I have 3-page information sites that pay more than 30,000 page data-fed sites with AdSense (although the latter do earn affiliate sales commissions).