This is one of many cases where the adage "Nothing is ever free" comes true. When you find a free puppy / kitten in the newspaper or street corner you don't know a lot about the animal you are considering to take home.
Have they had any vaccines? If so, when and what kind / brand? Because your vet want's to know, so they don't have to repeat anything. (So, I'm also having fun learning html code.)
Have they had a fecal exam or any dewormer(s)? What kind and when?
Do they have fleas? A heavy load of fleas causes anemia (blood loss).
Do those kitties carry leukemia, feline aids (FIV) or feline infectious peritonitis (FIP)? Leukemia they can be born with, FIV and FIP can spread through cat populations by sharing food and water dishes and fighting. And then there is the new calici virus. The old one was bad enough but the new hemorrhagic variety is scary because they die so quickly unless they are vaccinated.
Then there are the things you'll need to think about (there are expenses). Puppies and kittens are worm magnets and one dose doesn't cut it. There are many different types of worms and many different types of dewormers and most veterinarians recommend checking a stool sample before giving dewormers so you give the right thing. They are also like children; they are virus factories. The do have their mothers immunity for the first 5-6 weeks but then they are on their own unless you give them some help. This means vaccines need to start at about 5-6 weeks. And just like dewormers, one dose does doesn't cut it.
You know what goes into the adoption fee? The McCook Humane Society, has 4 paid staff members because they can't run on volunteers alone, there just aren't that many willing volunteers anymore. They feed($), water($), walk doggies, change kitty litter($), vaccinate($) and deworm($), medicate ($), help you pick out the perfect pet, get numbers crunched for the board of directors and are exposed to zoonotic diseases (those that both humans and people can get). It's not an easy job. If you are unwilling to volunteer time, they'd love it if you volunteered money, kitty litter, paper towels, bleach etc.
I've probably said this before but you might be better off getting puppies from a reliable breeder or the humane society. Then you'd get a puppy / kitten that doesn't have fleas, worms and has had at least one of 3-4 vaccines in that puppy / kitten series. Adopting a dog / cat / puppy / kitten from the McCook Humane Society you get a pretty sweet discount with our local veterinarians for vaccines, heartworm / leukemia testing and spaying and / or neutering plus a spay / neuter assistance rebate after the surgery is done (for adopted pets only). These future pets have also been socialized to lots of people and other animals.