Mahfuzar Rahman

The MailChimp plugin allows you to quickly and easily add a signup form for your MailChimp list as a widget on your WordPress 2.8 or higher site.
Not sure what <a href="http://www.mailchimp.com/features/full_list/" rel="nofollow ugc">MailChimp</a> is or if it will be helpful? Signup up for a <a href="http://www.mailchimp.com/signup/" rel="nofollow ugc">FREE Trial Account</a> and see for yourself!
After Installation, the setup page will guide you through entering API information, selecting your List and then add the Widget to your site. The time from starting installation to have the form on your site should be less than 5 minutes – absolutely everything can be done via the WordPress Setting GUI – no file editing at all!
<h3>Developer Mode</h3>
You can enable “Devleoper Mode” by adding the following line to your <code>wp-config.php</code> file just above the “That’s all, stope editing!” line.
<pre><code>define('MAILCHIMP_DEV_MODE', true);
</code></pre>
This will enable the MailChimp List Subscribe plugin to operate without the need to connect an external MailChimp Account, and will provide a 
subscription form widget that will not actually submit anywhere.
This will allow you to style and configure the widget in non-production environments that are not publicly accessible.
For more Developer Mode customization options see the following article:
http://connect.mailchimp.com/how-to/how-to-article-configuring-developer-mode-for-the-list-subscribe-wordpress-plugin

Easy Mailchimp Optin Form

The MailChimp plugin allows you to quickly and easily add a signup form for your MailChimp list as a widget on your WordPress 2.8 or higher site.

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.

Easy Mailchimp Optin Form vs Mailchimp List Subscribe Form