Currently the emails sent out by phabricator do not contain links that are clickable. This is annoying because I have to copy/paste the links into my browser each time (when reviewing a new diff revision for instance).
For the HTML version of the emails, you can simply use an <a target="_blank" href=""> tag, and for the plain text version, wrap the url in < > (i.e. <http://www.google.com>. This will cause email clients to properly pick up the link and make it clickable.