I received an email this morning from WordPress.com about its own ad system, an alternative to Google Adsense.
With Adsense, you embed a strip of html into your blog hoping that it matches your blog content. Relevant ads were rarely the case when I experimented with Adsense.
The WordPress ad system will be no more relevant but it may be easier to sign up and install. It's definitely worth testing.
Personally, the only reason I'd run ads on my blog is to raise money for charity. I'd rather sell eBooks if I wanted to monetize my efforts.