Inserts canonical link code into header. After activation, from your WordPress Settings, change Permalinks to Custom Structure: **/%postname%/
You will also need to update your Permalinks, which will give you desirable seo-friendly and user-friendly URLs anyways:
Some older versions of WordPress do not allow: /%postname%/ and you'll need to use: /%year%/%monthnum%/%day%/%postname%/
If you have an already established website and are applying seo-friendly URLs for the first time, I recommend using this plugin: http://wordpress.org/extend/plugins/advanced-permalinks/ so that people are automatically redirected to your new permalink structure and not an error page.
If you're changing permalink structure dramatically and already have well established PageRank on pages, you will likely lose it and have to rebuild it.
Since WordPress 2.9, they have implemented their own canonical link solution, it does not quite work the same as this plugin and has it's own flaws. If you're running the latest WordPress, but would rather use this plugin as your canonical link solution, you may want to compare, in my testing there didn't seem to be a conflict with the two, my plugin seemed to simply override the default canonical link.