The ones I'm most familiar with are: Acxiom, Datalogix, DataSift, Equifax, Full Contact, and Experian. There's always Twitter as well but they are mostly abstracted data and you can't just waltz in and get their firehose.
Most of these are pay to play. IIRC, when Facebook was creating their ads platform in like 06' or 07', they did a deep partnership with DataLogix where DL was effectively powering their targeting/user classification. That's largely why you still see dumb ad classifications (eg. Farmers vs. Non-Farmers) in the FB ads platform. DataLogix started out as a CPG play since they powered the "put your phone number in for groceries" platform that everyone used in the mid 2000's. They then started doing tons of data aggregations and purchased a whole bunch. DL become way more powerful when they were bought by Oracle in 14' since they could do identity resolution across their cloud. Eg. when you go buy a car, you are likely using the Oracle Auto suite which includes credit checks.
Most of these are pay to play. IIRC, when Facebook was creating their ads platform in like 06' or 07', they did a deep partnership with DataLogix where DL was effectively powering their targeting/user classification. That's largely why you still see dumb ad classifications (eg. Farmers vs. Non-Farmers) in the FB ads platform. DataLogix started out as a CPG play since they powered the "put your phone number in for groceries" platform that everyone used in the mid 2000's. They then started doing tons of data aggregations and purchased a whole bunch. DL become way more powerful when they were bought by Oracle in 14' since they could do identity resolution across their cloud. Eg. when you go buy a car, you are likely using the Oracle Auto suite which includes credit checks.