You can find some great options locally at VolunteerMatch. But this site also has a search function for “virtual opportunities” which you’ll see if you click on the tab next to the Local one. So if you live in the middle of nowhere and have nothing but time, this gives you the opportunity to give.
Do you like to read? If so, you may have heard of Project Gutenberg which produces free ebooks. This all happens with the efforts of volunteers. You can help them by doing some proofreading through their partner site called Distributed Proofreaders. There are many different ways to help and at many levels.
If you are a detail oriented person, here’s something to get involved with that will take as much time as you would like to give. The Smithsonian Institution has a need for digital volunteers at its Transcription Center. You will be helping them make historical documents and biodiversity data more accessible. They will give you the training you need and you can choose to work on a variety of projects.