The signing of a non-disclosure agreement (NDA) is a common practice at the workplace today — one is required to sign one when joining a new company/department, gaining access to advanced technology/methods and, of course, proprietary information. The use of NDA's is much less popular outside the working world. With a little thinking, however, NDA's can also be used in other situations.
At school, students are allowed to share ideas on answering assigned questions, but a student is expected to come up with a solution individually. There are worries, however, that someone would copy someone else's work word-by-word and present it as one's own. An NDA may come in handy in such a situation — one can still allow anyone to browse through one's assignment work, but no one (theoretically) can present it as their own work. For it to work, the signed NDA's would have to be presented to those grading the assignment before the assignments are collected, or else the markers would not know, in case two identical submissions are made, who copied from whom.
When gossiping between two or more people includes one providing the other(s) information about someone not involved in the conversation (and is known by all members involved in the conversation), the person providing the information may ask the people listening to agree to an NDA, especially when they talk behind the back of the one absent from the conversation. This way the news of the conversation would not reach the person being bad-mouthed. (Of course, it is not very practical to have others not disclosing what is being said, even verbally, in every conversation.)
There have been many court cases where a person files charges against one's ex-(dating partner or spouse) for releasing highly personal material (photos/videos/letters) to the public (usually out of anger/spite). While such acts are in very bad taste, (I think) there is little the courts can do, unless the material being released is highly revealing or damaging to the victim. By agreeing to an NDA at the beginning of the relationship, people may think twice about putting personal material of their partners in case the relationship goes sour. (I am sure that, at the beginning of a relationship, most couples would never imagine their relationship would end badly. I guess I have read too many stories where former lovers turn into mortal enemies.)
An NDA can also be used for happier circumstances — often people make plans to throw a friend or colleague a surprise celebration, only to have the surprise element removed by a slip of the tongue. To keep the person central to the celebration in the dark, those planning the celebration can agree to an NDA, so that no one is allowed to reveal anything to the person central to the celebration before the celebration actually takes place. (The punishment for breaking the agreement would likely be less severe than the situations above.)
While one may consider using an NDA in the situations stated above, it would probably complicate things by a lot. I have some ideas on making the use of NDA's in the above situations feasible and am willing to describe them in full, as long as those interested are willing to sign an NDA.