If you want to allow visitors of you blog to contribute posts, without giving them login information. TDO Mini Forms plugin allow you to add highly customizable forms on you blog. This plugin will allow non-registered users or subscribers to submit posts. Posts submitted by users are kept as “draft” until an admin publish these posts.

The plugin provides an extensive moderation view so administrators and editors can see posts awaiting approval and publish or delete them. Administrators can also ban specific users and IPs from using the form. Registered users have access to a “Your Submissions” page which lists their current submissions awaiting approval and links to their approved submissions as well.

You can get this excellent plugin form here