Mailchimp

Use the Mailchimp List Subscribe plugin to quickly add a Mailchimp signup form block, widget, or shortcode to your WordPress site.
After installation, if you already have a Mailchimp account, you’ll log in with that account and then proceed to configure settings.
If you don’t have an account, you can create one directly in the plugin. After entering in all your personal details, you’ll need to activate your account via an email that will be sent to you. Once done, you’ll proceed to configure settings.
On the settings screen, you’ll select your Mailchimp list, choose merge fields and groups, and configure other options. Once done, you can now add the block, widget, or shortcode (<code>[mailchimpsf_form]</code>) to your site. Typically, installation and setup will take about 5-10 minutes, and absolutely everything can be done via the WordPress Setting GUI, with no file editing at all.
WordPress.com compatibility is limited to Business tier users only. <a href="https://mailchimp.com/help/ways-to-add-a-signup-form-in-wordpress/" rel="nofollow ugc">How to add a signup form if you have a WordPress.com site</a>.
<h3>Access Token Encryption</h3>
Starting in version 1.6.0, authentication has changed to use OAuth. As part of this process, we retrieve an access token that can be used to make API requests. To provide a high-level of security, this access token is encrypted before being stored in the WordPress database. In order to ensure this access token can be decrypted when used, the plugin relies on certain security constants that should remain unchanged.
With no additional configuration, we use the standard <code>LOGGED_IN_KEY</code> and <code>LOGGED_IN_SALT</code> constants that are normally set in your site’s <code>wp-config.php</code> file. Some sites make use of security plugins that rotate these constants on a periodic basis. When this happens, we won’t be able to decrypt the access token and you’ll need to reconnect your Mailchimp account to generate a new access token.
To prevent such issues, it is recommended to define two additional constants in your site’s <code>wp-config.php</code> file: <code>MAILCHIMP_SF_ENCRYPTION_KEY</code> and <code>MAILCHIMP_SF_ENCRYPTION_SALT</code>. These constants should consist of a combination of characters, preferably at least 32 characters long. Once set, these values should not be changed. For strong values, you can copy some of the values from <a href="https://api.wordpress.org/secret-key/1.1/salt/" rel="nofollow ugc">here</a> and use them. You’ll end up with additional code like the following in your <code>wp-config.php</code> file:
<pre><code>define( 'MAILCHIMP_SF_ENCRYPTION_KEY', 'put your unique phrase here' );
define( 'MAILCHIMP_SF_ENCRYPTION_SALT', 'put your unique phrase here' );
</code></pre>
If these constants are added after you’ve already authenticated with Mailchimp, you will need to reconnect your account. To avoid this, you can copy the values from <code>LOGGED_IN_KEY</code> and <code>LOGGED_IN_SALT</code> (if they exist) to <code>MAILCHIMP_SF_ENCRYPTION_KEY</code> and <code>MAILCHIMP_SF_ENCRYPTION_SALT</code> respectively.
<h3>Upgrading</h3>
If you are upgrading to version 1.2.1 and you used the widget in your sidebar previously, all you need to do is drag the <code>Mailchimp Widget</code> back into the sidebar, visit the Mailchimp settings page (which will have maintained your prior settings), click the “Update List” button, and you’re done!

Mailchimp List Subscribe Form

Add a Mailchimp signup form block, widget, or shortcode to your WordPress site.

Concerted Action

A free WordPress plugin for the <a href="https://actionnetwork.org" rel="nofollow ugc">Action Network</a> online organizing tools maintained by <a href="http://concertedaction.consulting/" rel="nofollow ugc">Concerted Action</a>.
Features:
<ul>
<li>Create a WordPress shortcode or widget from any Action Network embed code.</li>
<li>Manage your saved embed codes using the WordPress backend. Supports sorting by title, type and last modified date, and provides a search function.</li>
<li>Modify Action Network’s default “thank you for your support” and “help us by using sharing tools” messages, and control which sharing options (social, email & embed codes) are displayed, using shortcode options or widget controls.</li>
<li>Use <code>[actionnetwork_list]</code> shortcode or Action Network List widget to show a list of current actions.</li>
<li>Use <code>[actionnetwork_calendar]</code> shortcode or Action Network Calendar widget to show a list of upcoming events. Optionally outputs upcoming events in JSON. Development of this feature was supported by <a href="http://www.thepeopleslobbyusa.org/" rel="nofollow ugc">The People’s Lobby</a> – if you like it, please consider <a href="https://actionnetwork.org/fundraising/donate-to-the-peoples-lobby" rel="nofollow ugc">making a donation to them</a>.</li>
<li>If you are an <a href="https://actionnetwork.org/partnerships" rel="nofollow ugc">Action Network Partner</a>, use your API key to sync all of your actions from Action Network to WordPress.</li>
<li>Create signup widgets which allow visitors to your site to sign up for your email list without using Action Network javascript embeds. This allows you to place a signup form on every page (for example in the sidebar), and still load Action Network embed codes for actions on particular pages (since Action Network’s scripts will only load one embed code per page). This feature does require the API key, so you have to be an <a href="https://actionnetwork.org/partnerships" rel="nofollow ugc">Action Network Partner</a> to use it.</li>
</ul>
Detailed specs for shortcode attributes, widget options, etc. are available on the Help menu for the Action Network page on the backend.

Action Network

Provides Action Network (actionnetwork.org) action embed codes as shortcodes and a calendar and signup widget

Mailchimp List Subscribe Form vs Action Network