This article is from the Problem Behaviors in Cats FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Mating cats can make an unbelievable amount of noise under your window. If these cats are feral, check with your local animal clinic about trapping and neutering these cats. Many will do them at little or no cost, depending on how many cats you're willing to bring in for the procedure. Eliminating the breeding stock in feral cats as much as possible will also help reduce the stray population in your area over time, and reduce similar problems like cat fights and spraying.
If the cats involved are owned by your neighbors, you might try a non-confrontational approach -- let them know what their cat has been doing and suggest that perhaps neutering their cat might help solve some of these problems. A politely worded note can be left on their door if you wish to avoid direct confrontation. You might eventually call Animal Control in your area for help, but first let them know you are having problems with their cat.