Assume that I created a free software that accidentally infringes on one or more software patents.
Can the holders of these patents sue me for damages, even though I did not make any money from the free software?
The patent holder may sue for lost profits and/or reasonable royalties. Wikipedia sums it up well:
Under 35 U.S.C. § 284, a patent owner is entitled to "damages adequate to compensate for the infringement, but in no event less than a reasonable royalty." Lost profits that result from infringement of their patent are also compensable.
Which by the way does not necessarily say that "normal" damages would not be awarded, but I'll leave that open because it's hard to predict anyways and the question is already answered with the above.