What is a natural and effective treatment for cats that have worms ?!

Try food grade diatomaceous earth -- wolf creek ranch has the best web site with full explanations of how it works, etc. It's non toxic and very inexpensive.
Also check out species appropriate raw food diet information -- cats who are not fed grains and other inappropriate foods almost never get parasites.
Someone is recommending shots? Get real. There are no vaccinations for "worms" or parasites, and all dewormers are highly toxic. You run the risk of making your pet sicker with neurological damage. And that goes for vaccines too.

Diatomaceous earth (food grade) is safe and natural. However, if you feed your cat a species appropriate diet of real meat, bones and organ meats the cat is healthier and can naturally fight off things like worms and fleas.

If the cat is infected bad right now use a traditional wormer then switch to DE to prevent reinfection. It's harsher but the presence of worms can do damage worse than the wormer can do.

Also, vaccines don't prevent worms and can even weaken a cat's immune system causing a reinfection of the worms or worse. There is no need to take a cat to a vet for worms.

Getting the cat it's shots from the vet. There is no over the counter thing or method that is going to really get rid of them.

Edit: You are most certainly free to explore all treatment options, but the truth is the best thing you can do for your cat is to take it to the vet, Or watch it die a slow painful death. I understand alternative medicine but it is not the answer for everything. This is one of those things where you need traditional medical help.

Worms *are* natural, as is sickness & death. (Nature kinda sucks.)

Your cat needs real medicine; take it to the vet ASAP.

Apple cider vinegar. Put a little in their water each day. It totally works!

Experience

take it to the vet and get the medication it needs for its size.