As per request from Jeanie from another thread:
Adopt a tiger (endangered tigers, our feline friends!)
Adopt a Jaguar (such beautiful cats too!)
Both the above is run by the WCS. https://wcs.org/
The Wildlife Conservation Society saves wildlife and wild lands through careful science, international conservation, education, and the management of the world’s largest system of urban wildlife parks. These activities change attitudes toward nature and help people imagine wildlife and humans living in sustainable interaction on both a local and a global scale. WCS is committed to this work because we believe it essential to the integrity of life on Earth.
Another good organisation is WWF, World Wildlife Fund: http://www.wwf.org/
There you can adopt a Panda and other animals (dolphins, elephants, wolves, etc) or just make a donation. It depends a bit on what country you live in what kind of adopted animals they have.
Support these organisations, they are worth it!