WordPress provides a basic level of functionality “out of the box”, commonly referred to as the WordPress Core. Themes and Plugins add to that functionality. Because only a handful are supported by the WordPress Core Team, quality varies widely. The most common issue is lack of updates to work properly with newer versions of WordPress.
I generally stick to the standard WordPress Themes, but I have used quite a few Plugins over the years. Though I do tend to keep it to a minimum for performance reasons.
Velvet Blues Update URLs – allows you to change all your links whenever you either change the URL of your site or import content from another site.
Shortcodes in Use – shows you what Shortcodes are being used on your WordPress web site and, when possible, where the Shortcode is defined.
Duplicator – when it works, which is most of the time, it is a great way to backup or move a WordPress site, changing its URL and/or web host.
Jetpack – the Site Stats feature provides exactly what my casual WordPress webmasters seem to want to see, and saves me the trouble of offering them Control Panel access to the web hosting companies I use.
Image Widget – easier to use than the standard WordPress widgets for placing Images in Sidebars.
Custom Headers and Footers – a simple way to create headers and footers on WordPress pages.
Update Notifications – more than 5 years out of date and generates warning messages when you have WP_DEBUG set to TRUE, but still sends e-mails whenever a plugin or theme has a new version available.
TinyMCE Advanced – adds the most commonly requested editing features, most notably Fonts, to the WordPress Page/Post Editor.
Display Kitchen Sink* – always displays the second line of icons in the WordPress Page/Post Editor.
My Private Site* – only allow registered users to view your web site.
Perpetual Calendar* – display day of week for any date; click here to test.
*Originally written by me but adopted out with the rest of the jonradio plugins when I could no longer support it.