WP Cleanfix is an useful plugin that allows you to check, repair and optimise your WordPress website. The first thing you should do before using the plugin is backup your database (note: it’s always advisable to backup your database before modifying your database in any way). You can optimise database tables, remove post revisions, spam comments and much more.
The plugin quickly highlights if there are any incomplete posts. It shows posts without authors, unused categories, attachments without posts and allows you to delete anything that isn’t attached. It’s also compatible with WordPress MU and you can find and replace content in posts, pages and comments (not sure why this feature is included in a clean up plugin but it’s a welcome addition nonetheless).
WP CleanFix isn’t the ultimate resource for optimising your database but it’s worth using every month or so to check if your database size can be reduced in any way.
WP CleanFix: Information & Download