A rottweiler. Hey, it worked for me, seriously!
But, I understand that not all want to go that route. First, most of the alarm companies make little or no money on installation. Indeed, many of them install the system for free.
They make cash on their own, overpriced, monitoring, which you are forced to buy via a two-year contract. Usually at a cost upwards of $50 per month or more.
You can install your own system. bit of a pain, but possible. You can also, most likely find some system that some firm will install for a reasonable price -- rather than "free" and then charge a reasonable monitoring fee.
Now, you don't have to have a system monitored. If you DIY the install, you can just have alarm sirens and flashing lights, no monitoring. Radio Shack used to sell a decent kit. I am sure you can find them around.
Or, you can do what I did, way back before the rottweiler. I got some "Protected by XXXX Security, 24-hour monitoring/armed response" and placed them around the house and drive. I had NO alarm system.
You can buy such signs on the Web.
Finally, this is exactly an issue that Consumer Reports writes about periodically. I would go to the local library and ask the people there how to look it up....or.....if your library is like mine, you can actually access all back (and current) issues via your home computer. IIRC, last time I read one of their reviews, they went through the whole thing. DIY, alarm companies, signs, alternatives to alarms (dogs), etc.