Two UFO's seen in official NASA released files during the STS088 Mission in 1998. 

NASA has dismissed the first image as space junk, possibly solar panel sections from a satellite. The second looks like it could be some kind of drone or weather monitoring device, however, I have no experience to base that assumption on.

Sources : (check the url if you don't believe the official nature of the images)

Image 1 : http://eol.jsc.nasa.gov/scripts/sseop/photo.pl?mission=STS088&roll=724&frame=66

Image 2 : ftp://eol.jsc.nasa.gov/ISD_highres_STS088_STS088-340-2_2.JPG