The Free WordPress Plugin hashtagger allows you to use #hashtags, @usernames (resp. @nicknames) and $cashtags in your posts.
The usage of @usernames can be activated optionally. @usernames can link either to the Users Profile Page or to the Users Website. If the username does not exist the text remains unchanged and no link is created. Optionally
@nicknames can be used instead of
The usage of $cashtags can be activated optionally. $cashtags link to the concerning stock symbol at MarketWatch, Google Finance or Yahoo Finance.