There was just a great discussion about this:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Pm26AAC
Good URL Tokens that will render as tracking links:
Token: {{my.URL1}}
Token Value: www.marketo.com
Token in HTML of email: <a href="http://{{my.URL1}}">Marketo Token URL 1</a>
Token: {{my.URL2}}
Token Value: <a href="http://www.marketo.com">Marketo Token URL 2</a>
Token in HTML of email: {{myURL2}}