The following are perhaps the most important texts of which I have not yet discovered any copies freely available online. Please feel free to contact me with further suggestions, or even better, with links to any missing texts in any language. Please note that, as it stands, this list is a subjective selection of what I think the most important gaps are. (Not everyone would agree with me, perhaps, that Malalas is an important omission.)
By far the most important gaps are in the areas of the Greek novel (in both Greek and translation); Greek lyric poetry (in translation); and Greek mathematicians (other than Euclid).
Please also note that I have neither the resources nor the time to make electronic copies of these texts from out-of-copyright editions myself; it is as much as I can do to keep the links in the catalogue up to date.