One my clients brought this plugin to my attention. Although running a multi-language site is a major undertaking, this plugin does a good job of simplifying the content management for a multi-language site. Some customization is required for other plugins and widgets to be integrated for full multi-language appearance, but if multi-language content is a hot item for your site, then this plugin may be worth a look.

Plugin Homepage
http://www.qianqin.de/qtranslate/

Once when working on two separate sites that would run off two separate domains, I was looking for a clean solution to have the two sites offer the same news. I knew the solution would be in using the RSS feed of one site to post to the other. There are a few aggregation plugins out there and after experimenting with them I found FeedWordPress to be the most flexible and easiest to manage. With a multitude of options, there are a number of uses for a tool like this. The best part is that when posts are created from a feed, the plugin attributes serveral aspects of the source in custom fields with that post. This allows you to develop your theme to attribute sources and provide links to the original content.

Plugin Homepage
http://projects.radgeek.com/feedwordpress

This is a great plugin that I stumbled across looking for a solution to the free/member blog sites. Have you ever wanted to have some content be free to the public and other content that is set for members only access. But at the same time, you want to streamline the process by which users can become members of the site and access that content. WP Sentry does the trick and then some. It builds off the built in functions of WordPress to organize users and content, but tweaks it to allow a group like subscribers to see private content while hiding any sign of the private content from the public. I see this being a great tool for groups that already have membership roles, such as churches, unions, civic groups, and clubs. But I have also had requests from bloggers looking for this kind of solution to help them monetize their content.

Plugin Homepage
http://www.peteholiday.com/wp-sentry/