there are outdoor motion sensors that use both RF and PIR to detect the presence of someone, they do a pretty good job at killing all the false alarms.. bugs, small critters, weather and light changes such as someone turning around infront of your house wont trigger them.
you can also use IR beam sensors like your garage door has as a safety device, if you have a front porch you could put a pair of those on the steps and get an alert any time someone passes through.
there are lots of things depending on what your needing, devices to detect vehicles entering a driveway, opening a mailbox, stepping on a mat, etc..
there are some more adavnced video motion techniques that would be decent for alerting, such as face detection.. but few cameras offer such features and I am unsure how reliable they are.