There are several free murder mystery scripts available on Party411.com, which include a variety of quirky themes that target different age groups. Most of the links provide files that include invitations that the party's host can print out and hand out to guests.
Additionally, Party411.com also offers some excellent tips for establishing a successful murder mystery themed party. These tips include how the host could direct the party and how to guide the other guests in their character parts.
MysteryCafe.com also provides links to free murder mystery party scripts with themes ranging from the Mad Hatter to the Old West.