Bernhard Kau

This plugin will show the first post with its full text but all the others with the text after the “more” tag

First Post Full Length

This plugin will show the first post with its full text but all the others with the text after the "more" tag

Wooliet

“Read More Right Here” uses the jQuery framework already included with WordPress installations. RMRH hooks into the WordPress creation of the 
“more” link and adds a custom class. When your blog is loaded, all links of this custom class are modified to no longer send the user to that post’s 
single page display when clicked. Instead, the WordPress database is queried for that specific post, and all content after the  tag (i.e. only what you need) is returned. 
The new content is then immediately displayed to the user, inline with the opening content.
For more information, please visit the <a href="http://www.wooliet.com/wp-plugins/" rel="nofollow ugc">Woolie T. WP plugins page</a>.

First Post Full Length vs Read More Right Here