Delhi Metro has one of the most practical implementation I have ever seen (I have a feeling that most IT projects are about hoodwinking the client, solving problems created by another IT solution). I am talking about the RFID based prepaid ticket cards. In most other metros I have seen, even if one has a prepaid card, one has to still go to a ticket vending machine or stand in a separate queue to purchase tickets for a particular journey.
The Delhi Metro card eliminates all that - just flash your card at the entry gate, and then flash it again at the exit gate of your destination - it gives you a 10% discount on the fare, as well as displays the balance available on the card. The beauty is that there is not minimum validity required while purchasing it, like, say, one month. Even if you are in Delhi for a day, you can get a card for Rs 100 (Rs 50 security deposit), use it, and then return it to get the full balance, including security deposit, even if you surrender it on the same day.