Prometheus
The Prometheus is the prototype of a multi-vector assault vessel, designed for deep space operations.  In combat, the ship splits into three parts, namely the top and bottom halves of the engineering hull with two warp nacelles each, and the primary hull which is also equipped with two small warp engines.  The single parts of the ship are heavily armed, furthermore the ship features regenerative shielding and ablative hull armor.

If the registry NX-59650 as seen in Voyager: "Message in a Bottle" is correct, this could point to a construction delay or even stop of more than a decade.  Furthermore, the Nebula-class ship Prometheus NCC-71201 shown in Deep Space Nine: "Second Sight" has a higher registry.  This is only possible if the ship has been destroyed or renamed in the meantime.  The new Star Trek Encyclopedia III, on the other hand, shows an MSD with the number NX-74913.  While this registry suits the ship much better, it could never be identified on screen and cannot be strictly canon.