Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Jonathan Thornburg,
"Event and Apparent Horizon Finders for 3+1 Numerical Relativity",
Living Rev. Relativity 10, (2007), 3. URL (accessed <date>):
http://www.livingreviews.org/lrr-2007-3
The date given as <date> then uniquely identifies the version of the article you are referring to.