Adobe Typekit fonts included with Squarespace
Squarespace comes with over 60 Adobe Typekit fonts that you can use for free in your Squarespace website (plus the full set of 600+ Google fonts). You can also use other Adobe Typekit fonts, if you purchase a license (instructions for doing this can be found on the Squarespace help site here).
Unfortunately, Squarespace doesn't supply a list of the Adobe Typekit fonts that are available for free. So I have made a list here for your reference. Please note that this list was correct at the time of research, which was 8 April 2014, so this list may have changed since then. If you notice any discrepancies, please do drop me a line so I can keep the list up-to-date.