Torre Argentina Cat Sanctuary
The mother of all cat sanctuaries around the world. Located in the historic center, they're open to visitors. Stop by and visit the local cats and please buy a souvenir or two. You can even adopt a cat at a distance. We've adopted two, Gottardo and Raptus. Nice people, too.
The Catboat - Poezenboot
Small "catboat" (houseboat) on one of the major canals right in the tourist center of Amsterdam. Nice people, dozens of cats, mostly looking quite healthy, and a gift shop for souvenirs to help support the cause. I don't know the severity of the problem in Amsterdam, but still a worthy cause.
This is a new support group started just about the time we got back from our first visit to Kotor, Montenegro, the "City of Cats". This is desperately needed due to the poor condition of the local cats. We were shocked and depressed at some of the poor strays suffering from a variety of serious conditions.
Nine Lives Greece
Their main aim is the humane reduction of the stray cat overpopulation through trap-neuter-return programmes. Each year, they get hundreds of stray cats in Athens neutered/spayed, greatly reducing the numbers of unwanted kittens born each season, and ensuring a healthier adult cat population.