aliciagh

BuddyPress Activity tab is used to output a list of sitewide, member or member’s friends activity. This plugin gets tags from each new blog posts activity and shows the most commonly used. 
BuddyPress Activity Tags doesn’t work with single WordPress installation or Multisite installation without BuddyPress. It must be activated for all sites using “network activate” in the Administration Panel. 
The widget configuration and tags style is based on <a href="https://wordpress.org/extend/plugins/simple-tags/" rel="ugc">Simple Tags plugin</a> by <a href="https://profiles.wordpress.org/users/momo360modena/" rel="nofollow ugc">momo360modena</a>.
Translations
The plugin comes with Spanish and English translations, please refer to the <a href="https://codex.wordpress.org/Installing_WordPress_in_Your_Language" title="Installing WordPress in Your Language" rel="nofollow ugc">WordPress Codex</a> for more information about activating the translation. If you want to help to translate the plugin to your language, please have a look at the bp-activity-tags.pot file which contains all definitions and may be used with a <a href="http://www.gnu.org/software/gettext/" rel="nofollow ugc">gettext</a> editor like <a href="http://www.poedit.net/" rel="nofollow ugc">Poedit</a>. 
Currently in the following languages:
<ul>
<li>Italian</li>
<li>English</li>
<li>Serbian (sr_RS) by <a href="http://www.webhostinghub.com" rel="nofollow ugc">Webhostinghub.com</a></li>
</ul>
Features
<ul>
<li>Click tags.</li>
<li>Customizable activity tags page URI.</li>
<li>Recent activity posts with a selected tag are showed in a page which contents the shortcode: <code>[bp_activity_tags]</code></li>
<li>Customizable style sheet for widget and results page.</li>
<li>Select how to display tags.</li>
<li>The size and color of each tag is determined by how many times that particular tag has been assigned to posts.</li>
</ul>
<a href="http://agora.grial.eu/pfcgrial/bp-activity-tags/" rel="nofollow ugc">See the plugin page for more information</a>.

BuddyPress Activity Tags

Adds a widget that displays a tag cloud with tags from new blog posts in BuddyPress Activity tab. Tested up to BuddyPress 1.9.2

dangrossman

The Calais Archive Tagger plugin automatically goes through your archives and tags every post you’ve written. The plugin uses the Open Calais API to perform semantic analysis of your post text and suggest tags. If a post already contains a suggested tag, that tag isn’t added, but other new tags found are. It takes about 5 minutes to tag 200 posts.
This plugin requires PHP 5 and the cURL library (both of which are available on most web hosts).
Also see <a href="https://wordpress.org/extend/plugins/calais-auto-tagger/" rel="ugc">WP Calais Auto Tagger</a> for suggesting tags as you write new posts.

WP Calais Archive Tagger

Goes through your archives and adds tags to your posts based on semantic analysis.

BuddyPress Activity Tags vs WP Calais Archive Tagger