As part of our service, we provide a links application. Basically, just a way to link to relevant content or sites. One of the features we added to this application was the ability for a site manager to automatically send an email to the webmaster of the linked site upon entry (regular users don't have the option, just managers). There is a email template that can be edited, and if the manager elects to send an email, the template is used when sending.
We have found this is a great way to both add a great resource (links) to your site and also get lots of reciprocal links in return. Site popularity is important, but so is getting traffic through mutual links.