How can I get my Mac to automatically connect to a given network whenever it becomes available? I take my Macbook out and about during the day and I'm often connected to my phone for internet access.  I'm always careful to not watch videos or start any large downloads until I get home for obvious reasons.  However, on numerous occasions I've got home, and started to use the internet freely, only to then realise I forgot to manually connect to my home (unlimited) network--that is to say I'm still connected to my mobile phone, blazing through my monthly allowance in no time.  

Is there any way to get my Mac to automatically (or perhaps with a prompt) disconnect to whatever network is was connected to, and connect to the new, designated network, whenever it comes into range?
It sounds like you may have the phone-hotspot network prioritized over your home wireless network. You can change this in System Preferences -> Network pane -> Wi-Fi service -> Advanced button -> Wi-Fi tab -> Preferred Networks list. Just click and drag to put your home network above the phone hotspot, and when it sees both it'll automatically choose the home network.

Note that this will not trigger a change whenever the home network comes into range, so it's not quite what you asked for. But it will re-scan and re-choose whenever it wakes from sleep, so if you put it to sleep on your way home (i.e. close the lid), it'll re-scan and choose the home network when you wake it back up.