A huge underground system that allows you to travel from one end of the city to the other for only 10 pesos. I wouldn't recommend travelling alone. It sometimes seems to be even busier than the streets and gangs of pickpockets do roam fairly freely among the beggars and salesmen who get on at one station and off at the next.
You need to buy a book of tickets in order to travel and even at the bigger stations it can be quite confusing to do so for the first time as none of the rail staff seem too willing to help out. Stick with it though and be careful.
Updates
Avoid the metro at rush hour and travel in the middle of the day or later at night. It's safe then. Avoid the VW Beetle taxis (LIBRE) and only use ones that you have to call first. On the metro, the first 2 carriages are supposed be reserved for women who can get groped at rush hour otherwise. The last carriage tends to become a gay cruising area, especially on the pink line (1). As of March 2006, it is only 2 pesos a journey on the metro.
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