The Transmatter Cube was a device located on the Justice League Satellite. A similar device was located at JSA Headquarters on Earth-Two. Both teams used the Transmatter Cubes in order to visit each other's respective realities for their annual JLA/JSA team-up.
It was originally shown to have been created by Lex Luthor of Earth-23 and according to him, the design came to him while he was on drugs. Lex Luthor of the main DCU seems to have created one himself as well, but this time he actually conceived the idea in a dream. While unfinished, it activated itself in the presence of Vyndktvx through which Superdoom appeared and attacked Superman.
Several TSA's appeared in different earths in the multiverse, most of them being stolen by the Legion of Sivanas as a way to communicate between earths and take over the DC Multiverse. Activated by the word SOS, the TSA has been useful and dangerous at the same time.