sitepointdevs

As descibed in-depth on <a href="http://www.sitepoint.com/sitepoint-random-hello-bar-wordpress-plugin/" rel="nofollow ugc">SitePoint</a> a hello bar is a thin bar of content that slides into view once the user scrolls past a set point on a page. 
The message content is up to you, but is ideal for advertising, product annoncements or other messages.
This plugin provides an admin interface to create multiple hello bar messages that can then be randomly displayed on user facing pages.
By setting weightings for each message you can determine how often each is displayed. On each page load a message is randomly selected 
(respecting the weightings given). A random number is generated client side before fetching the message content to ensure it is compatible 
with caching services such as W3 Total Cache.
All the required javascript and css is included in the plugin and can be set to enqueue via settings. 
Alternatively you can roll your own. To help with that the core javascript has been extracted into the <a href="https://www.npmjs.com/package/sp-hello-bar" rel="nofollow ugc">sp-hello-bar</a> 
npm module for you to include in your own scripts.

SitePoint Random Hello Bar

Randomly (with weighting) shows a hello bar message on page scroll.

Automattic

INN Labs

This WordPress plugin gives site administrators a way to insert widgets such as ads, newsletter signups, or calls to action into posts at set intervals.
The plugin registers a configurable number of widget areas.
The plugin can programmatically insert these widget areas into posts when the page is rendered. This setting can be overridden on a per-post basis via a post meta box or a filter.
If the programmatic insertion doesn’t work for a particular post, users can choose to place specific widget areas at specific spots in the post using shortcodes or blocks.

Super Cool Ad Inserter Plugin

This plugin enables the insertion of widget areas in your post's content via programmatic insertion at display time, via a shortcode, or via bloc …

SitePoint Random Hello Bar vs Super Cool Ad Inserter Plugin