Spay and neuter clinics across the country. My friend and I were actually planning all this out if I ever won the lottery (he knew I'd buy one sports car and a house, and then put the rest into a private foundation) and for the maximum impact, this seemed best. We'd work to get vets on board to do it at cost, and then the foundation would pay for it.
Why do so many rescue groups etc keep expecting veterinarians in private practice to provide spay/neuter at cost? They're private businesses owners and most really cannot afford to do the surgeries for less then they're already charging. And almost every veterinarian I know does some charity work already. As a whole, the profession already charges below cost for spays and neuters when you look at what regular surgeries cost. Based on the most recent data I've read on where the profit line is, giving anybody more than a 20% discount actually costs the office money. Why is the idea of a veterinarian being appropriately compensated for their years of education and investment in their practice such an evil concept?
Set up a trust fund for a major vet hospital to
1) research arthritis and cancer in dogs
2) have a mobile clinic providing free or heavily subsidised care for low socio-economic groups - you know, drive up and park and just deal with any and all for "what they can afford"
3) research laminitis and "cures" in horses
4) fund 3large animal vet students each year.
... and then provide a new premises for our local SPCA and take in a couple of senior dogs. After all, there is still room on the bed.