jeremyHixon

Gives theme developers and their users a convenient dropdown menu to preview the themes currently installed on the site as well as providing a download link for the one currently selected.
There is an options page in the admin section that allows you to:
<ul>
<li>Choose which themes are available in the select menu</li>
<li>Change the label text for the select menu</li>
<li>Enable/disabled the <a href="https://wordpress.org/plugins/easy-digital-downloads/" rel="ugc">Easy Digital Downloads</a> support.</li>
<li>Enable/disabled the bar globally. If disabled globally, enable on pages/posts.</li>
<li>Enable and disable the default stylesheet</li>
<li>Add custom content to the bar</li>
</ul>

Rational Theme Server

Plugin for theme developers to preview and serve downloads of their themes.

Melissa Cabral

A simple, lightweight plugin that displays useful technical information on pages and posts to aid in developing WordPress Classic themes. Provides quick access to things that are sometimes hidden, like post/page ID, slug, taxonomy terms, and post type slug. Theme Inspector tells you exactly what conditional tags are true on each view, and what template file loaded on each page view.
<ul>
<li>Use In Conjunction with the <a href="https://docs.google.com/drawings/d/1hJ0MpHO3HKBT5KsTpGtc_gDYZ5pi-HyxNcRtmPBBULE" rel="nofollow ugc">WP Template Hierarchy Document</a>.</li>
<li>Theme Inspector is only visible to logged in Administrators.</li>
<li>Appears on the right-side of the Toolbar (admin bar).</li>
</ul>

Theme Inspector

A developer's inspector to illuminate the WordPress Template Hierarchy and help with building WordPress Classic themes.

Rational Theme Server vs Theme Inspector