axenso

You can enable a tooltip that shows info about user/author when you rollover/click on related avatar.
Let’s speak clearly: if you are loooking for a plugin that enables the standard Gravatar hovercards, this is not the plugin right for you. Please look for another one.
When you rollover/click on an avatar (e.g. of a comment author), a tooltip popups and shows:
<ul>
<li>some additional links if the author is a registered user: e.g. to user post archive, to user external website, to user blog (if multisite)</li>
<li>some info from Gravatar if author is registered to that service: e.g. location, “about me” description, websites, other account pages (facebook, twitter….)</li>
</ul>
In plugin settings you can set up the javascript and css main options.
The plugin comes with some filter hooks, so you can customise the tooltip content at all.
The plugin uses <a href="http://craigsworks.com/projects/qtip2/" rel="nofollow ugc">qTip plugin for jQuery</a>.

Avatar Tooltip

To show a tooltip with info (member info and Gravatar profile) when you mouseover/click on an avatar.

ItayXD

tooltips are used to present a tiny amount of hidden content (mainly explanatory, so-called tips), that pops up when user moves a cursor over or clicks (less common) on a special target.
<h3>Key Features</h3>
<ul>
<li>It’s responsive. It relies on a maximum width value when viewed on large screens, adopts to narrow environments and picks the best viewable position relatively to the target (top, bottom; left, center, right).</li>
<li>It’s mobile-friendly. It pops up when a call-to-action button is tapped and disappears when tapped on the tooltip itself.</li>
<li>It’s HTML formatting capable. Need to write some words in italic or so? No problem, this will work out.</li>
<li>It’s extremely easy to use: A tooltip button in added to the default WordPress editor, all you have to do it click it and fill the pop-up dialog, the rest is taken care of automatically.</li>
</ul>
<h3>Advance</h3>
<ul>
<li>The button adds a WordPress short-code, if you want the tip to be HTML formated (avoid block level elements) you can just wrap it with [tooltip tip=””][/tooltip] in tinyMCE.</li>
<li>You can also assign the attribute rel=”tooltip” and title=”Enter your tip here” to any of body tags in HTML file where you want the tooltip to pop up when called.</li>
<li>You can change the pop-up look by editing responsive-tooltip.css. change it to what ever suits your website best!</li>
</ul>

Responsive Mobile-Friendly Tooltip

A WordPress plugin that helps you create responsive and mobile-friendly tooltip to present tiny amount of hidden content - the tip.

Avatar Tooltip vs Responsive Mobile-Friendly Tooltip