Pet Guardian Angels of America™ have compiled a comprehensive, ever-evolving list of financial aid resources for pet owners as well as shelter and rescue groups here.
Care Credit is a credit card for veterinary services offered by GE Money Bank.
SpayUSA has a short list of state resources for low cost spay/neuter programs.
Purchasing Pet Insurance can save a LOT of money on vet. bills.
Having Trouble Affording Your Pet? The Humane Society of the United States has a state by state resource list for assistance.