Peter Westwood

This plugin allows you to specify a specifc excerpt for WordPress pages.
The plugin adds an extra box to the set an Optional Excerpt for a WordPress page similar to the box which is available for posts. The box is aded using the dbx_page_advanced hook for versions of WordPress earlier than 2.5 and using add_meta_box() for WordPress 2.5 and later.

PJW Page Excerpt

This plugin allows you to specify a specifc excerpt for WordPress pages.

Andrew Ozz

Excerpt Editor provides a convenient place for creating and editing all excerpts. It can also add custom excerpts for pages (not possible without a plugin). Features: auto-creating excerpts when publishing new Posts and Pages, displaying customisable excerpts from the content for posts without an excerpt, appending excerpts from the latest posts to each Post or from the sub-pages to each Page, replacing the content for all archive and tags pages with excerpts (eliminates content duplication for SEO).
The excerpts can be used in the RSS feed, displayed on the Home or Archive pages and/or used for meta descriptions. The excerpts can be in any language supported by WordPress and the plugin contains .pot file to facilitate translations.

Excerpt Editor

Quick edit or create excerpts for both Posts and Pages, auto-generate and append excerpts or replace the content with excerpts.

PJW Page Excerpt vs Excerpt Editor