What are some real-life uses of inter-connected Arduinoes?

I am in temporary possession of 2 Unoes* and that made me wonder about hooking them together. It was dead simple to get one to send an H or L to the other and for the recipient to toggle a LED. It will be hardly more complicated to do that over two Wixels** which I'll do sometime this morning.

So my question: what sort of things do folk do with connected Arduinoes?

(*Got one for my son, he's collecting tomorrow. I should have got another one for myself.)

(**Wixels are quite cunning, for those of you who've never used them. If you plug one into the PC USB it gets power from the USB and switches to USB-to-Radio mode. Stick another on the Uno with power to Wixel's Vin and it switches to UART-to-Radio mode. That then works like a USB cable but wireless. If you have Wixels on 2x Unoes, each powered through Wixel Vin, then with both in UART-to-Radio mode you have a wireless connection between the Unoes. I'm going to test this, but afaics it's a wireless RxTx / TxRx crossover cable.)

Update: Did a quick connection between 2x Unoes with Wixels and they talk to each other as expected. Dead simple in fact. (It's a lot simpler than Pololu's documentation would lead one to believe, see for example my thread here.)