I found a really interesting virtual volunteering / online micro volunteering gig on Reddit: some of the community provide transcriptions of images on Reddit posts for those who rely on text-to-speech or other assistive software, and they are looking for additional volunteers.
Here the FAQ.
What a GREAT task for online volunteers - as is volunteers to transcribe videos, create alt tags on web pages, get rid of "click here" and "read more" links on web sites (changing them to descriptive links), all of which help tremendously regarding accessibility.