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.

DaganLev

huntlyc

Allows you to set global meta tags and customize on each individual page/post.
The plugin will generate the blog title and page title as default title tags if not specified otherwise.

Simple Meta Tags

Allows you to set global meta tags and customize on each individual page/post. Please Note: Does not support custom post types

Page Meta vs Simple Meta Tags