Here is a cursor to differentiate externals links of a website : clearest than a different layout, more visual than a precision in the title and less bulky than an icon next to.

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To select the external links, no need to add rel="external". Lachlan Hunt propose a tip who work with all modern browsers. I separate it in 2 part to obtain a valid code.

I select links starting with http:// or https, and I put the cursor. Then, I rescind the rule if the link starts with the address of my personal site.

a[href^="https://"],
a[href^="http://"]{
	cursor:url("external.cur"), default;
}
a[href^="http://delavy.hyperficiel.com"]{
	cursor:pointer;
}

Here is the cursor in png (5ko), and the psd file (26ko) for those who want modify it. I don't know the license for the cursors, so I put my creation in public domain