On one of the episodes of the British quiz series Quite Interesting they said that the Earth actually has two moons - the one we know and one called "Cruithne". Is that in fact a second moon?

Cruithne is a small asteroid moving on an orbit which is similar to that of our Earth. However, it circles the Sun rather than the Earth and hence isn't regarded as a moon. In-depth information about Cruithne (including an explanation of what its name means) can be found at http://www.astro.uwo.ca/%7Ewiegert/3753/3753.htm. There are other asteroids which move close to Earth, spiralling like a corkscrew around the Earth's orbit, as illustrated below for the asteroid AA29.

  

You may find this image as well as an MPEG animation of AA29's orbit on http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm.