The original Ninja Block comes with a RF 433.92 Mhz (433) dongle and future Ninja Blocks will have this baked in. 433 is awesome because you will find it in all sorts of low-cost battery powered devices, its range is pretty good too. That said, its not all sunshine and lollipops - there are good reasons its considered "low-tech" - only one device can "talk" at a time, labelling is appalling, all protocols are proprietary. So not everything will work out of the box, but a lot of stuff will.
The kinds of things that will work out of the box, are devices that send or receive single "commands". Think remote controls, garage door openers, movement sensors, etc. You Ninja up these devices by "recording" their command in the webapp and then either listening for or replaying those signals to do interesting things.
Stuff that works
Each original Ninja Block shipped with a PIR motion detector, an RF button and a magnetic door/window switch. We are working on making a bundle of these available through a reseller partner (we aren't in the business of shipping) but in the mean time you can purchase these very similar looking devices from Amazon - PIR, button, switch.
These are products we have directly used here at Ninja HQ:
Watts Clever Easy Off Sockets (available for all socket types). These are our preferred sockets. However, they are sometime a bit pricey, though we have bought them for less than $10 each. Jaycar 3 pack Single
Remote Control AC Power Outlet Plug Switch [Amazon] These work ok, especially for the price.
2 Outlet Outdoor Wireless Remote Control works well, comes in various brands on Amazon
For users in Germany, the uniTEC 48110 Funkfernschalterset's are reported to work well. You can buy them from Amazon here.
For users in the UK and Ireland, these remote power sockets work well. You can buy them here.
We have both the Swann Doormat and the Swann Driveway alarm and they work well. Strong signal and great old fashioned "bing bong". The doormat is especially handy as the signal is controlled with dip switches, so you can use the doormat without setting off the bell.
No-name PIR sensor. This is the one we ship with NInja Blocks, not attractive but it has Pet detection and works really well.
Stuff that DOES NOT work
Unfortunately some stuff doesn't work, or works really poorly. DO NOT buy these devices.
These sockets come in *many* flavours on Amazon, do not buy anything that looks like them, either the socket or the remote.
Devices with Protocols
Devices that send data require more effort to support. If we can understand their protocol we can implement it in the Arduino firmware and recognise them as devices. Thus far, we've only done this for La Crosse weather devices where keen hackers have already tackled the hard part of figuring out the protocol.
Stuff that works
The protocols for these two products have been implemented on the Ninja Cape. These were mainly a proof of concept, if there are other products that you would like to see, add them to this thread.