static-templates.php

<?php
/**
 * Plugin Name: Static Templates
 *
 * If most of your site content is in .php template files, and you're tired of
 * creating new pages, assigning them page templates, creating page templates
 * then doing it all over again on production, this plugin is for you.
 *
 * Examples:
 *
 * - /foo/          => /wp-content/themes/twentyten/static-templates/foo.php
 * - /foo/bar/      => /wp-content/themes/twentyten/static-templates/foo-bar.php
 * - /foo/bar/baz/  => /wp-content/themes/twentyten/static-templates/foo-bar-baz.php
 *
 * You're welcome.
 */

add_filter( 'template_include', function( $template ) {
	$url = parse_url( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
	$home = parse_url( home_url( '/' ) );
	$request = trim( preg_replace( '#^' . preg_quote( $home['path'] ) . '#i', '', $url['path'] ), '/' );

	if ( ! is_404() || empty( $request ) )
		return $template;

	// Loop throuh parts and makes ure they're sane.
	$parts = explode( '/', $request );
	foreach ( $parts as $part )
		if ( ! preg_match( '#^[a-z0-9-]+$#i', $part ) )
			return $template;

	// Using stylesheet directory so parent/child themes don't share static pages.
	$template_name = implode( '-', $parts );
	$template_path = get_stylesheet_directory() . sprintf( '/static-templates/%s.php', $template_name );

	if ( file_exists( $template_path ) ) {
		$template = $template_path;

		// We want trailing slashes, this is our last chance for a redirect.
		if ( trailingslashit( $url['path'] ) != $url['path'] ) {
			$url['path'] = trailingslashit( $url['path'] );
			$redirect = set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $url['path'] );
			if ( ! empty( $url['query'] ) )
				$redirect .= '?' . $url['query'];

			wp_safe_redirect( esc_url_raw( $redirect ) );
			die();
		}

		// We're hi-jacking 404s.
		status_header( 200 );
	}

	return $template;
});

add_action( 'template_redirect', function() {
	// Core will redirect /dashboard/ to /wp-admin/, we don't want that. Unless you do.
	remove_action( 'template_redirect', 'wp_redirect_admin_locations', 1000 );
});

Love this plugin. Saves me a lot of time. Have you tested with multi-site networks?

Please add a code before status_header( 200 ); to prevent error404 class inside body tag and Page not found inside title tag:

global $wp_query;
$wp_query->is_404 = false;