Alarm.com monitors your home alarm system via a wireless data network, negating the need for a landline. This can be especially useful for people like me that use Vonage and Skype because alarm system do not work well with VOIP. Monthly monitoring fees are $24.95, which is about the same as ADT, but Alarm.com allows you to receive e-mail or text message notifications about alarms as well as administer your system via their website. Looking at the self-install option for the equipment, the price is pretty hefty at $479 for the base unit, 2 window/door sensors, 1 wireless motion sensor, and 1 keychain remote; professional install options are also available.
Nice to know that a landline-free option is available, but you might just want to consider getting a pure-bread big dog for the price of the equipment.