OS X supports a feature called Hot Corners, in which moving your mouse to a far corner of your screen invokes functions such as window management features of Mission Control, starting or disabling the screensaver, and activating Notification Center.
To set this feature up, go to the Desktop & Screen Saver system preferences and click the Hot Corners button. In the panel that drops down, you can select an action for each of the four corners of your screen.
While potentially useful, Hot Corners are easy to inadvertently activate. For instance, if you have the top-left corner configured, then you might regularly invoke it by accident when accessing system menus, especially the Apple menu. It could be a good idea to assign that corner a relatively harmless action, such as preventing the screensaver from activating.
For a more decisive approach, you can take advantage of a hidden option, which is to require a set of modifier keys to be pressed in order for the Hot Corner to activate. To do this, when assigning a Hot Corner in the system preferences, hold one or more of the Shift, Control, Command, and Option keys and you will see the corresponding symbol appear next to the listed hot-corner action.
If you then select one of these actions, then you will need to hold the corresponding keys for that action to work.
While you can assign different modifier requirements to different corners, you can only assign one function to a particular corner at a time, so you cannot set, for example, the Option key to cause one action and the Command key to cause another in the same corner. These requirements only serve to prevent the Hot Corners from inadvertently being activated.