3 Added my own answer based on further research.
source | link

You cannot install any custom plugins on Wordpress.com for related posts. Unfortunately.

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.

As of June 2012, there is no feature built-in to Wordpress.com to display related posts.

You cannot install any custom plugins on Wordpress.com for related posts. Unfortunately.

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.

You cannot install any custom plugins on Wordpress.com for related posts. Unfortunately.

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.

As of June 2012, there is no feature built-in to Wordpress.com to display related posts.

2 misread question. updating answer
source | link

The one I've used and abused is called Yet Another Related Posts Plugin You cannot install any custom plugins on Wordpress. It's highly customizable, free, and has the widget that you wantcom for related posts. Unfortunately.

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.

You cannot install any custom plugins on Wordpress.com for related posts. Unfortunately.

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.

1
source | link

The one I've used and abused is called Yet Another Related Posts Plugin. It's highly customizable, free, and has the widget that you want.