Here is the flea page from the Companion Animal Parasite Council (CAPC): link
. CAPC is comprised of parasite specialists and I would consider them one of the best sources for parasite information.
As a veterinarian, I agree with some of what has already been said. Basically, research the products available because they are not all the same. There are FDA approved products which specifically evaluate the safety and effectiveness of the product on animals. These tend to be prescription only, but not all are. This does not mean that they are completely safe, but they have been investigated. There are also EPA approved products and these are generally OTC (over the counter). EPA evaluates products differently and is generally more concerned with the impact of the drug on the environment. Again, this does not mean that they are not safe products, but the evaluation criteria are different.
My personal recommendation for fleas on a cat would be Advantage, which happens to be an EPA product. In my experience, it is very effective and has few concerns. Frontline is also a good product. Whatever you choose, make sure it is safe for cats. Some dog products can be lethal if applied to cats accidentally.
Revolution, which was mentioned above, is a prescription product and is effective against other parasites. My personal experience is that it is not as effective against fleas. It also may be more than you need. You should also treat all animals in the household.
One last thing is that flea infestations may lead to tapeworms. Cats that groom and eat fleas will complete the life cycle for tapeworms. They typically look like grains of rice in the feces.