I finally broke down and got Minecraft, even though I think Minetest is fairly excellent and getting better. Even though you pay Mojang for the game, making it polished takes at least a couple of mods and making it really slick takes a few more. I wrassled around with minecraft on Linux and found a decent set of bits which make the game pretty fancy. This guide assumes you've got minecraft successfully installed, but not too much else.
I've got an Android phone with, currently, no phone service. Well, OK, it's a WinMo phone chainloading Android, but that's relatively irrelevant. Soon I will want to avoid using minutes from my incoming T-Mo PAYG phone card, so I'll still want to make phone calls using SIP any time I'm in WiFi range. You can even get out of paying for outgoing calls so long as you turn them into incoming calls.