She plays Willow in "Buffy the Vampire Slayer"
Go to google.com and enter the keywords "Alyson Hannigan Sex Tape" and you will see hundreds of sites offering downloads of the tape.  Trouble is IT IS NOT ALYSON HANNIGAN.  It is Luna Lane,  born 21 November 1981 Los Angeles, California an adult film actress who began working in the porn industry in 2001.  The movie was made for exploitedteens.com.  Members noticed a resemblance between the actresses and the scene began to appear on P2P sites as Alsyon Hannigan.