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.