This basic role has some limitations, such as the inability to add images or to access posts for editing once they have been submitted.Other roles give more access, but may also give contributors capabilities to alter other aspects of the site, which you may want to avoid.
1) Another option for managing roles and capabilities is the WP User Control plugin. This adds a login, registration and password reset widget to your sidebar.
downside: users may not want to register in order to contribute to your site
2) Submit by email: Use a contact form plugin – almost any one will do – and point it to the WordPress post by email address. Experiment with your form design so you get the right fields for post title, content and so on. more info here
downside: have to do some tweaking to format the posts in the right way for the site.
3) Use other more robust plugins:
A) User Submitted Posts has multiple input fields (Author Name and URL, Post Title, Tags, Category and Content and an image upload field) which you can customize for your needs. User submitted posts (or links or news) are automatically saved as drafts. You can also set them to publish immediately or after users have a certain number of approved posts. There’s a lot of admin control over the posts and you can filter these posts easily in the dashboard. The downside – you can only display this plugin via template tag or changing your theme design. Download this plug-in here
B) If you want users to submit relevant news in your sector, then FV Community News is a good choice. This plugin creates a form from which users can submit a title, link, description and image. This content then goes to your admin panel for moderation or editing before you make it live. You can change field names and the default behavior for user submitted content and can place the form via widget or shortcode. There are additional options for producing a list of posts and a tag cloud for community content. (WordPress download link)
C)The WordPress Guest Post plugin allows you to add content to your site via a form on a purpose-built page. The form includes a Captcha to help weed out spam. These posts go in their own tab in the admin dashboard and you can review them before posting. The latest version of this beta plugin is available here.
update: not fully supported, seems beta and with some bugs.
D)Gravity Forms isn’t free, but thousands of users rate it as one of the top contact forms out there. So you won’t be surprised to find that it’s a great option for getting UGC on your site. All you have to do is set up the form as usual, but use form field titles from WordPress for title, content, tags, categories and so on. GravityWiz has an excellent tutorial on the process to help you get started.
more info: http://www.webdesignerdepot.com/2012/03/wordpress-plugins-for-user-submitted-user-generated-content/
For images: http://wp.tutsplus.com/tutorials/allow-users-to-submit-images-your-site/
info on videos:http://premium.wpmudev.org/forums/topic/looking-for-an-integrated-seamless-user-generated-content-video-solution-for-wordpress-multisite-and-buddypress-that-is-so-easy-to-use-my-grandma-cou
via this article: http://blog.crazyegg.com/2012/09/07/user-generated-content-wordpress/