I think sponsors and plugs for your subscription service make sense. Maybe dial down the free content a little and beef up the subscription content to encourage people to sign up (and pay) for your premium services.<p>One warning about sponsors, though: Sometimes the sponsors may expect more than a simple display ad for their money -- they'll want "rich media", special ad units, or interaction with your audience that may end up being more irritating than Google AdSense.<p>Another suggestion: If you haven't already done so, set up an email newsletter or alert system based on interesting stats, user-determined criteria, or sudden spikes. That could be one of your premium services.