A related links plugin that allows you to manually specify links, even if they're on another site. Requires PHP5.
This plugin allows you to manually enter links that are related to a post. They can be on any site and you can enter just the URL or an entire link (specifying the title attribute, onclicks, etc). There are helper functions to display them in your theme or you can simply tell the plugin to automatically add them to posts or even just single posts. The list is output as an unordered list (ul) with an optional title, and everything has CSS classes to make styling easy. Requires PHP5.