Here’s a simple method. Add a mailto: link to the button that looks like the following.

mailto:EMAIL-ADDRESS-GOES-HERE?subject=My subject line goes here&body=I just downloaded a case study that you might find interesting - Acme Industries conquers the world. You can view it at tinsert a link].

Spaces are fine in the subject and body, but don’t use HTML characters like =, ? or >. When the link is clicked, the to: address is EMAIL-ADDRESS-GOES-HERE, which reminds the person to insert an address.

Hey guys, you should take a look at AddThis which allows you to embed social sharing icons, but also your standard “send to a friend” button that opens a window like this:

They also have an API if you really want to get into the nitty gritty and customize it…