A Dead Drop or Dead Letter Box is a technique used by spies to pass information between two contacts without compromising either party. There are numerous variations on the technique, and new ways of performing the dead drop are always being found.
One good example of a dead drop can be found in the movie Enemy of the State. In this movie, in order to contact the private investigator/former NSA operative Brill, the protagonist is told to draw a line with chalk on a mailbox, and then leave the package with the data to be sent underneath a specific seat on a ferry. The chalking of the mailbox acts as a signal to the contact, letting him know to check the dead drop, and by placing the material in a place where it is unlikely to be looked for (and where the receiving contact has private or semi-private access) it's safety is insured - and neither contact knows the appearance or identification of the other.
Another example of a dead drop can be seen in the TV show The X-Files. When Mulder needs to contact X he marks his window with tape in the shape of an X. This signals to X that Mulder desires a meeting at the predeclared location.
al-Qaeda operatives working in Britain came up with a method using an e-mail account to pass information - by sharing the mailbox password among the contacts, messages could be left for each member of the cell by writing the message and saving it as a draft - this way the e-mail never was actually transmitted and therefore could not be intercepted.
The downside of most dead drop ideas are that many times the data must be left in a public or semi-public place - and therefore can be compromised if the area is under surveillance.