This doesn't look like damping off to me. I'd suspect the gnats (which it's their grubs in the soil that are the real problem. However the seedling pictured looks quite good. For tomatos just plant them with their stem buried and they'll root on the stem itself. You didn't specify if the damage is on other seedlings or the severity so I can't give a reasonable response. The drooping seedling probably (unfortunately) is a goner.

mauch1~
thanks for your response. There is widespread damage on most of the tomato plants, but oddly enough, not on the flowers, peppers, etc.

But the seeds were bought from many different seed suppliers, otherwise I would suspect a seed quality issue.

I only ever saw one gnat. Is this damage typical if what gnat grubs do?

The only other thing I can even think of is that maybe the damage was done by my sprayer, as I was reaching to spray the different plants close to soil level? When you said this did not look like dampening off, I tried to consider what else it could be.

Thanks again!