ASI has launched common tickets that can be used across all its monuments. You can buy these tickets in many places, such as airports, tourism department offices and a few hotels besides buying them on location. The ticket is valid for a year and can be used at any monument.
There are three varieties of such tickets. One can be used for all world heritage sites. The second one can be used at all other ticketed monuments, while a special ticket is available for Taj Mahal alone.
With this, if you are planning to travel a lot and are likely to visit places where ticket counters can be crowded, you can save the hassle of having to stand in a queue and buy a ticket at each monument. This could be especially useful for Taj Mahal, where you normally see large queues at the ticket counter.
For more details, see http://timesofindia.indiatimes.com/city/patna/Now-no-waiting-for-tourists-to-get-entry-tickets/articleshow/5302485.cms or http://www.asi.nic.in