First things first: Try reasoning with the renters next door, and if that doesn't work, try the same approach with the landlord. As strange as it may sound, they may not realize that their noisy parties are disturbing you. Nolo's book Neighbor Law: Fences, Trees, Boundaries & Noise, by attorney Cora Jordan, has excellent, straightforward tips on negotiating through neighborly quarrels. However, if you get no results, you have stronger courses of action available to you.

If the cars are parked on your property, they are being parked there by trespassers. A call to the local police station ought to take care of that problem.

As for the noisy parties, what you have here is a nuisance, and a potentially serious one if you have reasonable grounds to fear for your safety. One way that neighbors have successfully dealt with this kind of problem is to sue the owner -- not the tenants, but the owner -- of the offending house in small claims court. This kind of lawsuit is very effective -- especially if you can get other neighbors who are annoyed by the ruckus to join in. For more information, see Nolo's Small Claims Court Resource Center.