BackUpWordPress - Automatically creates daily, backup zip files of your entire WP install. Broken Link Checker - Checks your blog for broken links and missing images and notifies you on the dashboard if any are found. Microkid's Related Posts - Display a set of manually selected related items with your posts Remove Comments URL - Removes the URL field from the comments form. Keeps comment spam low.
The list varies according to one's needs. The best choice is to go to install new plugins and click the popular plugins section. It will show a list of all those wordpress plugins that are most sought.