Hi Kate,
I think the object you may be interested in is the Minor Planet (Small Solar-System Body) 3753 Cruithne.
http://en.wikipedia.org/wiki/3753_Cruithne
It isn't so much a "Moon" as a resonant co-orbital asteroid. These objects are not uncommon in our solar-system and Jupiter has 100's of them.
Another body with somewhat similar orbital properties is 2006 RH120 which is now confirmed to be an Apollo booster stage that is (for the most part) in a heliocentric orbit but sometimes becomes a temporary 2nd "Moon" of Earth.
http://en.wikipedia.org/wiki/2006_RH120
Hope that helps.
Best,
Les D