You can take a quick glance, the way you'd preview an attachment before opening it in an editor, and if you're using Mac OS X Mail on its own screen you don't have to switch desktops, monitors and applications.
Open Links to Web Pages Right in Mac OS X Mail
To open a web page right in Mac OS X Mail from a link in an email (without going to your browser):
- Move the mouse cursor over the link you want to open.
- Click the down arrow that appears to the link's right.
- You can also click on the link with the right mouse button (or tap on it with two fingers), then select Quick Look URL from the menu.
Note that you will get a slightly shrunk display of the web page perfect for a quick glance; you will not be able to interact with the page in Mail directly, though. To open the page in its full and functional glory, click Open with Safari or follow any link on it.
(Updated July 2011)