I'm looking for a software like
StackTracker that allows to track a specific tag on
StackExchange sites. Do you know somes?
Have a look to StackGuru, it's awesome.
StackGuru is a Jabber/XMPP bot which gives you a command line interface to Stackapps sites, including Stackoverflow, Serverfault and Superuser. The bot allows you to follow questions tags by giving you a notification each time a new question is ask on the followed tag. You can also follow a question which will send you a notification each time someone post a comment or an answer on the question.
What about just using the RSS feed for the tag?
If you click on a tag, then the RSS feed link is down the bottom on the left.