Steve

This plugin uses custom fields to allow the page title tag to be customised and differ from the actual page title.
Both meta descriptions and keywords can also be added if required.
Page meta details can be modified on any publicly accessible post type, such as posts and pages, as well as custom post types.
Theme and plugin developers should note that it is possible override the page title within your templates by setting <code>$wppm_title = 'Newly defined title';</code> before the call to <code>get_header();</code>.

Page Meta

Adds the ability to override the page meta title and add in meta descriptions and keywords for pages.

svil4ok

Get tags on every single post (is_single()) and put them in meta keywords tag in header (wp_head()). Otherwise it’ll use the keywords define in $default_keywords. Get the_excerpt() and put it in meta description tag in header (wp_head()). Otherwise it’ll use the description define in $default_description.
Requires the wp_head() tag !

Page Meta vs Meta Keywords & Description