I think it's Clea DuVall - she was a recently a guest on Grey's Anatomy in the episode
Who's The Actress In The NEW Zyrtec Commercial?
Megan Paul
... Davis in zyrtec commercial? Who is the actress in the new zyrtec commercial? Who is the girl in the indooroutdoor zyrtec commercial?
wiki.answers.com/Q/Who_is_the_girl_in_the_recent_zyrtec_commercia...
That is the lovely Megan Paul.