Let's consider a similar scenario. If you made a beverage which poisoned a number of people, would you be absolved of liability because you gave it away for free? Of course not.
As there is no contract between you, they would have to bring an action against you in the tort of negligence or negligent misstatement OR under consumer protection law.
To succeed at tort they would need to prove that you owed them a duty of care; from Donoghue v Stevenson "You must take reasonable care to avoid acts or omissions which you can reasonable foresee would be likely to injure ... persons who are so closely and directly affected by my act that I ought reasonably have them in contemplation ...". Most cases will founder on your inability to foresee the use to which your software may be put.
Consumer law is jurisdiction specific but they generally contain warranties that what you provide (gratis or otherwise) is fit for purpose, merchantable and that you do not make false and misleading statements. There is a chance that a case brought under this sort of law could succeed as you have not limited the purpose, specifically declared that it is not of merchantable quality and have (presumable) said what it does so that, if it doesn't do what you said, you have been misleading and deceptive.