Bell Wire, "is a low-voltage wire designed for use in interior residential applications", commonly used to connect alarm systems, door bells, buzzer systems, intercoms and thermostat controls. It is a 2 x 20 gauge wire (commonly red/white or black/white). Bell Wire is also my favorite DIY wire to use for home automation (Raspberry Pi / Arduino), for connecting up sensors. It is fairly cheap and available locally at both Home Depot and Lowes. It is still small enough to connect to bread boards (barely), very flexible, easy to strip, and a good conductor for long runs.
My other favorite, but more expensive, is CAT5/CAT6 wire. I use CAT5/CAT6 when I need to run multiple lines to the same location, like with my home security panel project.