Hacking
Hacking is the act of seizing control of an already-built Block. This block may have been built by another player, spawning on an NPC Ship, or built by you yourself before losing ownership (such as after respawning in a respawn ship outside of a Medical Room). Hacking itself is the same regardless of who built it originally.
Hacking involves using a Grinder to remove components from a block until all the Computer components have been removed. If everything is then welded back on using a Welder, the computers will be re-configured to recognize the one doing the welding as the owner. The hacking threshold is shown by the blue line on the construction interface; once the blue line has been crossed, the block has been hacked, and welding can begin anew.
There is no need to use new computers. Simply re-weld the ones you gathered from the device. It goes without saying, but devices without computers cannot be hacked (and as a result, those blocks do not deny access based on ownership). To see a complete list of blocks that incorporate computers (and are thus hackable), see the Computer page.
Be warned: when hacking starts the block being hacked will attempt to broadcast a warning to its owner using an Antenna. Be prepared for a vengeful Engineer coming to protect his property!