For some time, iOS has offered wifi setting on a per-network basis as to whether your device should automatically join a network. Maybe you sit down at a coffee shop once to grab something, but you don’t always want your device automatically joining when you walk by or frequent said shop. This setting is for you.
Apple has brought this option to OS X Yosemite. Go to System Preferences > Network > Wi-Fi, and if you’re connected to a hotspot you will see the option I have pictured (and enabled) here.
I’m not sure if this setting is present for all networks, or just known public hotspots. It appeared for ‘attwifi’ at a Starbucks, but disappeared when I tethered to my iPad. Anyone know for sure?