Wi-Fi control for SLRs is not that common, and the official tools are often expensive, so why not consider the XSories Weye Feye, a remote control for SLRs that enables a lot of features that wouldn't be possible on most cameras. For a start, there's a live video feed from the camera, "touch to focus" support, which you don't get on most SLRs - especially older ones, like our D90. Plus you can set up timelapse photos too, which is amazing, considering what the equipment to do this sort of thing can cost. The Weye Feye is £200, has a built-in rechargable battery and a slick app for iOS and Android. More features are promised in the feature.
Read our hands-on review for more information.