In order to show which users are currently viewing my application I used the following method. First I created a table in my databaser with a column called "username". Then whenever a user logs in to the application his username gets added to the table. To show users who are working currently in my application I just query this table. when user logs out his username automatically gets deleted from my table which is fine. But when a user leavs my application without loggin out he is still in my table despite he has closed the application. For example Yahoo also has such problem in its messenger. When a user gets disconnected from the internet he or she is looks on line but only after some time the user becomes offline.
What is the best method to eliminate such problem?

Each time they access the page, update a field which tracks the time they last visited; then in your list of users, you can query only those where that time field is less than five minutes ago. (or whatever duration you think is best)

The data will still be in your table, but will be filtered out by the query. If you don't want all that old data clogging up your table, you may want to trigger a regular program to clear out the old records. Otherwise, you could just leave it there, and overwrite it when the same user does eventually come back.

Hope that helps