WA001 in Boeing parlance - and not really a prototype, just the first 777 built.
Boeing had always intended to deliver the number 1 aircraft, but then it was discovered that they hadn't built the wing correctly - some spars were put in backwards or something along that line. The error didn't affect the ultimate strength of the wing, but it changed the load distribution and hence affected the fatigue life - so the assumption was Boeing would never sell it and just use it as a flying test bed (as was done with the first 747, although technically that aircraft was a prototype and it would have been impractical to ever deliver it). However apparently Boeing did revised fatigue calculations that said it was suitable for commercial service with suitable life restrictions which allowed it to be sold for commercial service.
The first 757 and 767 were never delivered to commercial service - they stayed on as flying test bed aircraft and were eventually used for military prototype work - the 757 used to validate F-22 FBW software, the 767 as an optical test bed for tracking ballistic re-entry vehicles.