[petuniasevan]

Yes, that's true - in a way. However, this "second moon", actually asteroid 3753 Cruithne, is not in a permanently stable orbit. For now, though, its quite elliptical orbit (quite inclined with respect to the orbital plane of the Solar System) has a resonance of 1:1 with Earth, and its oscillation period is somewhere around 760-770 years. However, the orbital stability projections begin to break down in 5000 years and completely destabilize by 10000 years.

A good site for java illustrations of this asteroid and its orbit, compared to Earth and other planets, is this LINK.

In addition, there are now two more near-Earth asteroids known to be currently in resonant states similar to those of Cruithne. These are 1998 UP1 and 2000 PH5.

[RightWhale]

Cruithne

That is the one. It is a candidate for early asteroid mining, although it is uncertain if it could be a carbonaceous chondrite. If it is, it's better than a gold mine. If not, it's just an iron mine or a science outpost.