The LifeLine Grant Program at United Animal Nations provides funding to animal rescuers, pet owners, and non-profits for emergency veterinary services they otherwise couldn't afford. You can find a list of other assistance organizations on their site, UAN.org, under Programs/LifeLine Grants/Other Assistance Programs.
Certain breeds of dogs are typically less desired than others, particularly pit bulls and retired racing greyhounds. Go to Pit Bull Rescue Central (PBRC.net) or The Greyhound Project, Inc. (Adopt-a-Greyhound.org) to educate yourself, adopt one, or donate.
Operation Baghdad Pups (BaghdadPups.com), a program organized by the Society for the Prevention of Cruelty to Animals International, rescues stray dogs and cats befriended by soldiers in Iraq and Afghanistan and transports them back to the US to live with the returning vets.
Groups like Neighborhood Cats (NeighborhoodCats.org) help humanely control the feral cat population with a trap/neuter/return (TNR) program. Once returned to their colonies, the cats receive regular care in the form of food and shelter.