I just installed a SimpliSafe DIY alarm in my home and really like it. Easy to setup and it's really inexpensive compared to the professionally installed systems. It uses a cell signal to contact a call center and then send police. I also put a water sensor in the basement.
My only complaint is there is no provision for Mac users to be able to change advanced settings. It's a serious flaw for them to bring a product to market without Mac support, however they can change settings for you over the phone.
Otherwise it's great. If anyone wants a 10% coupon, let me know and I can hook you up. I would get a discount as well.