It probably depends upon your niches, audiences and goals (building authority vs. readership vs. keyword carpet bombing). I suspect searching article directories for ideas will get old quickly. Most of the folks who submit articles to the directories tend not use much creativity to uncover topical areas.

In some niches (especially those with mass appeal), you can probably use authority blogs for ideas. If your goal is gaining readership or reader participation in your blogs’ comments area, this can be a great way to measure what folks are responding to on other blogs.

For example, if you’re running a personal productivity blog, you can check zenhabits.net or lifehack.org for ideas.

Obscure niches will be more difficult, of course. Presuming you’re not an expert in an obscure niche, you may need to call a manufacturer, grab some catalogs, subscribe to the niche’s magazines, etc.

Hope that helps light a creative spark.