There are multiple Tor clients that I used and they seem to connect to the Tor servers. Tor servers seem to show good performance, despite many spammers hide behind the Tor and all communication passes through the servers several times to obusficate you. The traffic must be huge. Who pays for all that fun?
I pay for it, I run a relay.
You too could help cover the cost of providing the service to users if you have the resources to spare, some time to work through some documentation and the inclination.
From the "What is Tor?" answer on the Tor Project FAQ:
Tor is a program you can run on your computer that helps keep you safe on the Internet. It protects you by bouncing your communications around a distributed network of relays run by volunteers all around the world: it prevents somebody watching your Internet connection from learning what sites you visit, and it prevents the sites you visit from learning your physical location. This set of volunteer relays is called the Tor network. The way most people use Tor is with Tor Browser, which is a version of Firefox that fixes many privacy issues. You can read more about how Tor works on the overview page.
The relays are run voluntarily for various reasons by similarly aligned organisations, individuals and people who benefit from Tor usage.
If you too would like to volunteer, you may be interest in the "How do I decide if I should run a relay?" FAQ entry.