The Earth does in fact have a kind of "second moon", called Cruithne. It is really more of a small "Co-orbital Near-Earth Asteroid" similar to Phobos and Deimos around Mars, exept without a real stable orbit. (There are another two minor cousins, 1998 UP1 and 2000 PH5)
It doesn't cause any differences in tides etc. and is invisible to the naked eye, but you might be interested in checking out:
Earth Usually Has More than One Moon | Space for general info,
Cruithne, Earths "second moon" for some orbit simulations, (kitekat already posted those, sorry for the repetition) and maybe
http://www.campusprogram.com/...edia/3/37/3753_cruithne.html if you want some of the detailed stats.
Hope it interests you!
This message has been edited by Jex, 06-19-2004 05:48 PM