eharmony is free to sign up and free with the Basic Membership.
With a Basic membership you can review any matches and filter within the Match Preferences you specify once you complete the Compatibility Quiz. In addition, you can view primary profile information about unlimited matches, send and receive Smiles and Icebreakers, as well as respond to a first message at no charge. Furthermore, you can browse anonymously and see who has recently visited your profile.
While there are benefits to being a Basic member, such as no time limit under this membership, there are far more advantages when you upgrade to Premium. You can find a more detailed description of the memberships here.