Any online service or application can integrate with the U2F protocol. Google and Dropbox, as well as others, have integrated their solutions with U2F YubiKeys. See Google Drive for Work and Dropbox for Business.
One of our key partners, Duo Security, was one of the first to offer enterprise server solutions supporting U2F. See Duo Security and U2F.
RC Devs has also created OpenOTP™, an enterprise-grade user authentication solution based on open technologies. See Open OTP Authentication Server.
For a complete list, please see our enterprise integrations page for a complete list of solutions that have enabled FIDO U2F, https://www.yubico.com/support/partners/#FIDO-U2F