I've been using Gmail as my non-work email client. I have it set up so mail to my domains comes to me there. It's a vast improvement over Yahoo. I told myself that I would endure the ads and never click on them. But really, they have been useful.
I've recently had some behind-the-scenes changes to this site so I could have a couple of extra domains, and Subversion and Trac installations (more about that another time) for running the WP Interop project and in the process of that happening I was temporarily unable to login. But, while I was sorting out the access problems Gmail showed me an ad for a competitor of the ISP with whom I was corresponding. Maybe it read my frustration. That was very instructive, 'cos the competitor's site was down completely. I persevered here.
The other incident is not actually to do with ads, but with the Related Pages Gmail suggests. I have been corresponding with someone about Word to XHTML possibilities and Gmail suggested I read Holes in Microsoft Office XML. This was really useful because I thought I was subscribed to the O'Reilly weblogs via Bloglines, but I'm pretty sure this one never made it to me.