Can we show status of user in his signature/profile so that we can understand whether he is Online or Offline?? If it is not already implemented can you provide this feature?
You can already tell (approximately) when they were last seen on the site by viewing their profile.
These sites are more about the questions and answers than about individual users, so I don't think we need this indicator in the signature link.
There is already such feature. If you go to User profile, there is a property "SEEN". This shows the minutes passed after the user was last seen.
If the minutes is within 10 or 15, generally the user is still online.
IGNORE the below answer based on comment from Grace -
According to my observation, there is one more way too. If user has an accept rate, then the accept rate is shown in green if he is online, else it is shown in grey. Not sure if this is really true or not.