If non-commercial flights count, I was on an Intel corporate shuttle flight back in January with I think maybe 5 passengers on it. This was after a lengthy delay, and most people decided to just cancel their trips and go back to the office. As it turns out, those people made the right decision. As this was a Friday morning, most people (myself included) were making a day trip up to Oregon. The flight was supposed to be
MHR-
HIO, but I never actually made it there. After being delayed due to conditions at
HIO, we were finally told that while
HIO was still fogged in, they were going to try for
PDX and bus us over there. So the few of us who decided to wait it out boarded the ERJ-145. When we got closer to
PDX we were told the visibility there was still below their minimums as well (No Cat III approaches for them, apparently). So we were in a holding pattern for a while and then diverted to
RDM. After we landed two other Intel shuttle aircraft diverted there as well, one from
SJC and one from
AZA. The combined passengers from all three flights hung out in the FBO while some of the pilots went over to Subway and bought lunch for everyone, as by then it was around noon. After much discussion and confusion about what was going to happen, it was finally decided that (probably for positioning purposes), one of the planes would return to
SJC and one would fly to
HIO (by this point it was afternoon and the fog had all burned off) and we were welcome to go with them on either flight. I figured at that point by the time I got to
HIO and from there to the office I would barely have time to do anything before it was time to go home, so I called reservations and asked for a seat on the
SJC-
MHR shuttle and got on the plane headed for
SJC. By the time I got back to
MHR and from there to the office it was after 5:00, an entire workday gone.
Now for commercial flights, I can't recall any that had an extremely low load. A Qantas redeye from
DRW to
SYD comes to mind as possibly the lowest, maybe a 50% load factor on a 738. I managed to get a section of 3 seats to myself, curl up with 3 pillows, and actually get some decent sleep on that flight.