A lot of people have mixed feelings about this, because of the potential risk to all of the kitties involved. Personally, I think it's a wonderful thing. Two days after Sasha had her litter, John came home and told me about 4 kittens whose mother had been killed and the people didn't know how to care for them properly. By the time we got them, they were in pretty bad shape and we lost two. The babies started out as bottle babies, but after a couple of days Sasha "adopted" the two remaining kittens. They are healthy and wonderful kitties now at 10 weeks old (you may have seen them in my previous posts, Captain Squishy & Garfield).
Then last night I got a phone call from a lady who had found two kittens abandoned on her porch. They are close to the same age as my current foster cat's kittens... so we took them in and Lady adopted them immediately. They're actually starting to eat on their own (their choice) but it's good for them to have a momma cat to teach them how to be cats and keep them clean and give them "snacks" of milk.
One tip though, if you do bring in foster kittens, put her current kitten in with the new arrivals for a little while so they'll all smell similar to the momma... I haven't actually had to do that, my momma cats just accepted them right away... but not all cats will accept new kittens.