Here is a list of all default ownerless ships in X3: Terran Conflict. In X3: Albion Prelude, ownerless ships exist as well, but their locations are randomly chosen in each game.
Abandoned ships may be claimed by space walking and accessing the ship information screen or use of System Override Software. The ships listed below usually spawn at the listed location at the start of each game. Many players use these ships as a seed for expanded operations either adding them to their fleets or selling them for credits.
The coordinates are given in meters relative to (0, 0, 0) of the sector. An easy way to find them is to use the "Move to Position" command from the autopilot.