Blucube

If your site has an SSL certificate you might find that you can access the site via both SSL (https) and non-SSL (http) URLs. This is a bad idea for security, and for SEO, as it can look like duplicate content on different URLs.
The answer to this is to redirect requests to non-SSL (http) URLs over to their SSL (https) equivalents using something called a 301 redirect. This tells the client (and search engines) that the resource they are looking for should always be accessed over SSL. This plugin offers two methods to achieve this:
<ul>
<li>By intercepting WordPress pages at header time, and if they are not already being requested over HTTPS sending a 301 redirect header, or</li>
<li>By adding mod_rewrite rules in the .htaccess file to redirect all requests to their HTTPS equivalents using 301 redirects.</li>
</ul>
Optionally, this plugin can also set <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security" rel="nofollow ugc">HSTS</a> headers for you, and make sure that all requests use the same hostname (i.e. fixing the issue where many sites can be accessed using both www. and non-www. URLs).

Simple SSL Redirects

Lightweight plugin to ensure access via SSL/HTTPS. Uses 301 (permanent) redirects for SEO benefits. Optionally sets HSTS and forces canonical domain.

Simon Waters

TLS is growing in complexity. Server Name Indication (SNI) now means HTTPS sites may be on shared IP addresses, or otherwise restricted. For these servers it is handy to be able to set desired HTTP headers without access to the web servers configuration or using .htaccess file.
This plug-in exposes controls for:
<ul>
<li>HSTS (Strict-Transport-Security)</li>
<li>HPKP (Public-Key-Pins)</li>
<li>Disabling content sniffing (X-Content-Type-Options)</li>
<li>XSS protection (X-XSS-Protection)</li>
<li>Clickjacking mitigation (X-Frame-Options in main site)</li>
<li>Expect-CT</li>
</ul>
HSTS is used to ensure that future connections to a website always use TLS, and disallowing bypass of certificate warnings for the site.
HPKP is used if you don’t want to rely solely on the Certificate Authority trust model for certificate issuance.
Disabling content sniffing is mostly of interest for sites that allow users to upload files of specific types, but that browsers might be silly enough to interpret of some other type, thus allowing unexpected attacks.
XSS protection re-enables XSS protection for the site, if the user has disabled it previously, and sets the “block” option so that attacks are not silently ignored.
Clickjacking protection is usually only relevant when someone is logged in but users requested it, presumably they have rich content outside of WordPress authentication they wish to protect.
Expect-CT is used to ensure Certificate Transparency is configured correctly.

Security Headers

Plug-in to ease the setting of TLS headers for HSTS and similar

Simple SSL Redirects vs Security Headers