Which gets better results for you? I'm using AdSense, but YPN is looking like it might be a competitor. I'd like to know what you think. Also, had any good luck with WidgetBucks?
YPN is a good competitor to AdSense - probably the best actually, but it's limited to US residents only and that doesn't look like changing anytime soon. WidgetBucks is doing well at the moment - 20c for each US/Canada click. The main problem is the slow (2 or 3 seconds) load time for the widgets - many people navigate away before the widget attracts their attention.
I can't use YPN because I'm in the UK. I know a lot of US webmasters who consider YPN the most profitable alternative to AdSense - they're very similar in terms of format and payout.