Black holes: one of the most (theoretically) dangerous things in the universe. They consist of highly concentrated matter at a single point, such that the gravitational force exerted by the black hole is so great, even light cannot escape. However, this isn't entirely because the escape velocity is greater than the speed of light. Some astrophysicists believe that the major reason light cannot escape is because the mass of a black hole is so concentrated that it warps space around it such that every path leads towards the center of the black hole.
In addition, the high gravity of a black hole causes some time dilation. In theory, if we could get close enough to a black hole to experience the effects of time dilation, but not so close that our escape velocity would be too great to leave orbit, we could utilize a blackhole to create forward time travel. The major problem, however, would be getting back afterwards.