While the big jets are what Boeing is known for, they have a number of other aircraft that they use for their own purposes. I haven’t got all of these by any stretch of the imagination but I have come across a few at various times. They have BBJs that they use for executive transport. They also have some Bombardier Challengers that are able to promptly get people from A to B.
If you are looking a bit more locally, there is at least one Cessna Caravan that is used for various duties. I am not sure what its role is exactly but I imagine it is a handy way of getting people around the northwest and it can probably also move parts up to a certain size if needed.
Another runabout is a Northrop T-38. This can be used for chase duties but I also suspect it is a crew hack since it seems to make regular runs between BFI and Moses Lake without crossing paths with any of the test aircraft. There are also T-33s used for chase work but, sadly, I have get close to any in action. Just a distant overflight shot. Hopefully I will see them before too long.