How to allow user login only once at a time? I want to allow a user login to the site only once at a time. User should not be able to login at other machine/place while he is logged in.
I want to user password should get destroyed once used and should promt user to "request new password".
I also want to show user login time,logout time, number of visits, last visit time...etc in a block for user.


How can i get these in Drupal?
You can get what you're looking for with a combination of existing modules:


Session limit module
Password policy module
User stats module + Views module


You shouldn't have to write a single bit of code using those 3 modules.

You don't mention which version of Drupal you're using but each of those modules have D6 and D7 releases (albeit a dev release for one of them).