The WP-PostRatings plugin allows you to allow visitors to rate, like etc your posts.
How to use:
Activate the WP-PostRatings plugin in the plugin section of your dashboard, more details on the Plugins page.
A ratings section is added to your blog’s dashboard. There is a sub section where you can set the type of rating that you want.
After you have set this up you just add [ratings] to the posts or pages you want users to rate. It will appear like this: