There is no such widget currently available on the free-hosted WordPress.com blogs.
For others interested in that plugin available only for wp.ORG, here is some info:
We cannot use plugins here:
To move to wp.ORG you must have a domain name (which you buy) and a host for your site (Netscape Solutions, godaddy, or a multitude of other choices), and software to make your site function (which could be the free software from wp.ORG) .
If you have a blog here on wp.COM then it is not difficult to export your wp.com site to wp.org:
Here are a few details about the differences between wp.com and wp.org: