The Dawn spacecraft has three ion engines that are mounted such that their thrust vectors are at significant angles. You can see this in this image:
In the image, one of the three engines is active. Two questions:
1) Why three engines? I suspect this is for redundancy, but seems very wasteful. In addition, several other missions use a single propulsion system.
2) Why different thrust vectors? I suspect this has something to do with redundancy and making sure that the thrust vector passes through the center of mass.