Artifakt Digital

The artifakt Hashtag Plugin allows you to easily and quickly make keywords 
searchable across your website, in the same way it does on other websites where 
hashtags are used. Just adding the # symbol in front of a word or phrase makes 
it a link to the archive page of that hashtag.
Originally this plugin simply created links to a search results page for the 
hashtags, and that worked well but we got to thinking about SEO and how it 
would be nice to index the hashtags on your site. So we created a custom taxonomy 
“Hashtags” (duh) so now you have some options on how you display the archive 
pages and the SEO meta for the pages (if you are so inclinded). You could even 
create a tag cloud of all your hashtags using WordPress’ tag cloud widget.
We are leaving all the customizing of the archive pages and SEO work up to you 
and your theme but your options are much more plentyful since version 2.0
Have fun! And as always, please let us know if there are any issues or 
features you’d like to see.

The artifakt Hashtag Plugin

This plugin makes #hashtagged keywords searchable across your website.

Darkseal

IMPORTANT: This plugin requires Jetpack’s Publicize module: read below for additional info.
As the name says, this plugin will automatically append hashtags to any content sent by Jetpack’s Publicize module. This means that the plugin will work only if the Jetpack plugin is already installed on your WordPress platform with the Publicize module set to active. Don’t worry, tho: if any of these requirements are missing this plugin won’t do anything, without breaking your blog or compromising its performance in any way.
Plugin Features
<ul>
<li>Hashtags will be created using the post <code>tags</code>, (optionally) trimming the space chars, until the maximum character length is reached (if any).</li>
<li>You’ll still be able to manually add custom hashtags, pre-pending or appending them to the auto-generated ones just like you always did.</li>
<li>You can reorder the hashtags as you like, regardless of how they were added.</li>
<li>Dupe check included, ensuring that the same hashtag won’t be included multiple times – unless you manually do that.</li>
<li>Automatically added hashtags won’t break the 118-character limits of the social message text to keep the message length under the standard tweet + link limit.</li>
<li>The above character-length threshold can be changed and/or set to infinite with ease.</li>
</ul>
Useful Links
<ul>
<li><a href="https://plugins.svn.wordpress.org/publicize-with-hashtags/" rel="nofollow ugc">Plugin SVN Repository</a> hosted by WordPress.org (in real-time sync with the GitHub repo).</li>
<li><a href="https://github.com/Darkseal/publicize-with-hashtags" rel="nofollow ugc">Plugin GitHub Repository</a> (in real-time sync with the official SVN repo).</li>
<li><a href="http://darkseal.github.io/publicize-with-hashtags/" rel="nofollow ugc">Plugin GitHub Page</a>.</li>
<li><a href="http://www.ryadel.com/works/publicize-with-hashtags/" rel="nofollow ugc">Plugin official page</a> at <a href="http://www.ryadel.com/" rel="nofollow ugc">Ryadel.com</a>.</li>
<li><a href="http://www.ryadel.com/en/add-hashtags-to-jetpack-publicize-shared-posts/" rel="nofollow ugc">Installation & Usage Guide</a> at <a href="http://www.ryadel.com/" rel="nofollow ugc">Ryadel.com</a>.</li>
</ul>
<h3>Authors</h3>
<ul>
<li>Darkseal of <a href="http://www.ryadel.com/" rel="nofollow ugc">Ryadel.com</a>.</li>
</ul>

Publicize With Hashtags

great plugin and works out of the box!
but please explain how to modify $mess_max_length and $strip_spaces without modifying your plugin file?!
we need some filters here. and please add a do_action hook for adding some custom hashtags by default. i.e. i have a post_type called "cars" and with each post, i want to add "#car" without tagging it each time by hand.
something like this would be nice:
line 92 of publicize-with-hashtags.php add:
do_action("hashtags_append");
then in my functions.php i can write:
function hashtags_append_static(){
	global $mess, $post;
	if(get_post_type() == "cars") return $mess .= ' #cars';
	return;
}
add_action("hashtags_append","hashtags_append_static");


please add apply_filters and do_action hooks

Automatically append hashtags to content sent by Jetpack Publicize module. Hashtags will be created using post tags. Dupe and length check included.

The artifakt Hashtag Plugin vs Publicize With Hashtags