FamilyTree DNA – https://www.familytreedna.com
Original company in US to offer DNA testing. Remains the only company to offer gender-based tests (Y-chromosome and Mitochondrial DNA). Results can be uploaded to other DNA sites.

Ancestry.Com – https://www.ancestry.com/dna/
Has largest DNA database and sells kits. The results of tests done through Ancestry can be uploaded to other DNA sites. But Ancestry does not accept uploads of DNA results from other sites. The database is searchable by members.

MyHeritage – https://MyHeritage.com
Sells kits for testing and has searchable database for results. MyHeritage DNA results can be downloaded.

Gedmatch.com – https://www.gedmatch.com/
A universal database for DNA results. Testers can upload their results from other sites and be able to look for matches from many sites.