Style External, PDF, and Mailto links using CSS and a background image. For a great set of icons to choose from visit http://www.famfamfam.com/lab/icons/silk/.

a[href^="http"] {
padding-right: 15px;
background: url(external-image.png) no-repeat center right;
}

a[href^="mailto:"] {
padding-right: 15px;
background: url(email.png) no-repeat center right;
}

a[href$=".pdf"] {
padding-right: 15px;
background: url(pdf.png) no-repeat center right;
}