During the course of an interview, an interviewer may ask whether you have any other offers. Being asked this question is generally a signal that things are going well. Interviewers ask this question both as a means to assess your decision timeline and to reinforce their opinion of you as a strong candidate. It is both natural and understandable for firms to want to know if they have any competition in getting you. Your stock would likely go up in their eyes if you disclose an offer from a strong competitor of the firm you are interviewing with.
Consequently, candidates lacking any other offers may feel tempted to respond to this question with a strong "Yes." However, one should never do this as the next question will invariably be "Who do you have an offer from?" The recruiting world is smaller than you might think. People move around and might have unexpected connections. It's not advisable to ever lie on an interview. See this story about a job candidate named Jeffrey Chiang, who apparently lied about having an offer with Bank of America when he was on an interview with Morgan Stanley. Unfortunately, someone checked on his story and his name and story are now becoming fairly well known.
http://dealbreaker.com/2009/10/jeffrey-chiang-will-be-receivi.php
For further advice on answering this question, see this link http://www.gottamentor.com/viewAdvice.aspx?a=284