Every piece of open-source software needs donations of some form (time/money/hardware/etc). Donate to the ones you use. If they accept your money, great. Many only accept time though (in the form of patches etc).

Tor needs help with software too.

Tor relies on exit node diversity for security. Jurisdiction diversity, organisation diversity, sysadmin diversity, hoster diversity, hardware diversity, OS diversity etc. Funding exit nodes reduces a lot of those. It would be best to setup a node yourself and convince people you know to setup nodes. If you still want to fund nodes, the Torservers project accepts money to do that. Tor itself had/has an experiment on funding exit nodes. Exit nodes tend to attract law enforcement in some cases, there are no guarantees. Relay nodes are much safer.

http://torservers.net/ https://lists.torproject.org/pipermail/tor-relays/2012-July/...

Tor needs funding diversity too, hence their recent individual supporters campaign.

https://blog.torproject.org/category/tags/funding

Tor FAQ about funding:

https://www.torproject.org/docs/faq.html.en#Funding