[eluser]pistolPete[/eluser]
There is probably no "plugin" for that task, but it's easy to implement.
Add a column "hidden" (ENUM: yes, no; default: yes) to your comments table; a new comment is then saved with hidden="yes".
On the public pages, only display comments where hidden="no".
In your admin area add a controller / method to change the value of the hidden column from "yes" to "no" to approve the comments.