Many school around the world have PC's in their labs which let you do a telnet throughout the world without needing any account or password, or ID to enter the school. There are plenty of different types of room keys. These programmable keys will always be reprogrammed if a guest checks out. On the other hand, if you go to the reception and claim that you lost your key, they will always program a spare key for you. This way you could easily get into someone else's room.
Some hotels have quite fancy light systems.
This should help saving energy while you are gone, but sometimes even the air condition will turn off, so you have to fool the box with a paper or spare key. Some systems will turn on certain lights just when you insert the key into the door and open it. This is quite unfortunate if your roommate sleeps while you go cruising and clubbing at night. When you return, the light will shine bright and wake him up. The only thing that helps is unscrewing the light bulbs.