I say Plugins very loosely and I mean plugin, hack, mod list. So what do you think are THE definitive plugins for WordPress? Which are the plugins that you simply could not do without? Are there any that are hidden gems that I have not noticed? Please limit your suggestions and links to WordPress version 1.2 and above only.
What new plugin, when and if developed, would become the definitive plugin for WordPress (spam panacea notwithstanding)?