The important thing to remember here is that product pages are Drupal nodes. There are innumerable modules out that that work to add features to nodes, some of these being node recommendation modules. The following modules enable e-mail recommendations and sharing through other social bookmarking sites and web services. For an example of all three working together, check out the green share link at the end of this blog post from Top Notch Themes.
- Forward - http://drupal.org/project/forward
- Share - http://drupal.org/project/share
- Service Links - http://drupal.org/project/service_links
See this thread in our forums for more information.