stranger in sidebar.

Site URL?

Though no infection or malware shows up in security scans, what you describe seems to be adware/hack, especially as you have not inserted that banner. You may click on it when you find it and see which site it is linking to.
These are the standard workaround links for hacks that you may start working with:
http://codex.wordpress.org/FAQ_My_site_was_hacked
http://wordpress.org/support/topic/268083#post-1065779
http://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
http://ottopress.com/2009/hacked-wordpress-backdoors/

Anything less will probably result in the hacker walking straight back into your site again.

Additional Resources:
http://codex.wordpress.org/Hardening_WordPress
http://sitecheck.sucuri.net/scanner/
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html

Try deactivating ALL plugins and switching to twentytwelve theme. Looks like you have some mark-up errors on that page:

http://validator.w3.org/check?uri=http%3A%2F%2Fsurviving-a-breakup.com%2F&charset=%28detect+automatically%29&doctype=Inline&group=0