Our system is not fancy or elaborate, but it works for our situation. I use halter buzzers that go off when the mare lays flat out. For our mares that like to lay flat out just in general, the buzzer seems to make them choose an alternative way to lay down until they are really in labor. We also have Swann Nighthawk cameras that carry video and sound. They have worked very, very well for us over the years. The night vision on them is exceptional.
For us, I hadn't opted to get a more expensive mare monitoring system because I am able to arrange my schedule to be on site pretty much when I think I need to be during foaling season. If I'm at work, and Harvey is at work, we are 50 miles away so online cameras or a pager still makes it at least an hour before we'd be here. Because I can control my office hours w/ clients, simple buzzers and cameras with sound have worked well for us.
These cameras are also good if you have a sick horse to monitor visually. Saves trips to the barn! Plus, the real fun begins once the foals are born. I couldn't count the number of times Harvey and I have laughed out loud at watching one of our foals playing in the stall at night!!!