The Free WordPress Plugin hashtagger allows you to use #hashtags, @usernames and $cashtags in your posts.
This plugin uses the WordPress Tag system to field your post under the desired tags. The plugin follows the rules for #hashtags on Twitter, Facebook and Google+.
The usage of @usernames can be activated optionally. @usernames can link either to the Users Profile Page or to the Users Website.
The usage of $cashtags can be activated optionally. $cashtags link to the concerning stock symbol at MarketWatch, Google Finance or Yahoo Finance.