Ipsos, CNN, AP/NORC all use this type of panel. They are difficult to do at the state level because of the number of people needed, and congressional district is impossible. But for national, these are great. So don't just write off ALL online polls. Know which ones.

But yeah, it's pretty gross out there methodologically once you get beyond the main players. YouGov is the only pure opt-in that I'd trust. One more thing - there are some hybrid models as well - Verasight has a hybrid probability/opt-in method that I've used and liked.