You won't necessarily be identified as a Tor user if you donate by credit card, only as a Tor supporter: Some people recognize the Tor project as a valuable organization and choose to support it financially, even though they don't use Tor themselves.
One option you have is to donate to an organization which supports Tor. That way, if you use a credit card or Paypal, your donation will be linked to that organization rather that to the Tor Project directly.
The Freedom of the Press Foundation is currently (at the time of writing) collecting donations to "Support Encryption Tools for Journalists". Of four recipient organizations, one is the Tor Project, another is Tails; you can choose how your donation will be divided up.
The Freedom of the Press Foundation also accept anonymous donations by mail.
Torservers.net and its member organizations also accept donations by a variety of methods; this money is used to fund fast Tor relays and expand the Tor network.
The above organizations also accept Bitcoin donations, however Bitcoin is not anonymous unless you take adequate precautions.