We all see things that at first blush don't make sense. Catch a glimpse of something out of the corner of your eye. Movement where you didn't expect it. Sometimes you know that it's just junk on the windshield, and a little Windex will take care of it. Other times, there just is no good explanation.
If you think your sighting rises above the level of "just junk", the folks at The National UFO Reporting Center may want to hear from you. If you've seen something within the past week, and can give information regarding time, date, location, and a description of what you saw, they'd love to hear from you at their hotline at 1-206-722-3000. The line is staffed 24 hours a day. They ask that if want to report an incident more than a week old, that you go to their website to fill out their online form or get their postal address to mail a report in.
These guys are serious about what they do here, so please don't submit any kind of joke or hoax report—I'm sure they've seem 'em all and won't have any problem just deleting the bogus stuff.