We do have an advertising program called WordAds, which you can read more about at http://wordads.co/
Advertising outside of WordAds at WordPress.com is not allowed. For full details, please see:
http://support.wordpress.com/advertising/
http://wordpress.com/types-of-blogs/
If you require other advertising, you might be interested in a self-hosted (WordPress.org) installation. There are no restrictions on advertising if you setup a self-hosted WordPress.org blog instead of a WordPress.com blog, but there are some added responsibilities.
Please see this link to learn about the differences:
http://support.wordpress.com/com-vs-org/
We have hosting recommendations at the following link if you are interested:
http://get.wp.com/hosting/