preview on LAN
In order for Construct 2 to share its preview over the local network, you must start it with administrator permissions on Windows Vista and newer. To do this, right-click its shortcut or EXE file and select Run as administrator. You'll have to do this every time you run Construct 2 when you want to preview on the LAN.
You'll see a security alert about allowing administrator permissions to Construct 2. Click Yes.
If you're running Construct 2 from Steam, you may need to do this another way. Try locating the Construct2.exe file (by clicking 'Browse local files' in Steam's properties for Construct 2). Then right-click the file in Windows, select Properties, and in the Compatibility tab there should be an option to Run this program as an administrator. Check the checkbox and then the next time you run from Steam it should run as an administrator.
Now Construct 2 is ready to preview on the LAN. Open a project and preview it. (If you're not running Construct 2 with administrator permissions, you may see an error message here - try going back and starting Construct 2 with administrator permissions again.) Notice the address bar should now have your local IP address, e.g. 192.168.1.3:50000, whereas before it would have been localhost:50000.
Now, get another device on your LAN. It can be a phone, tablet (like an iPad), a laptop, or another desktop connected to the same LAN. Enter that address to the device's browser, e.g.:
http://192.168.1.3:50000/
You should now see your preview running!
Try changing something obvious in the project and preview again. Both your PC's browser and the second device should simultaneously refresh, showing the new version of your project. This can make it really quick to test games on devices like iPads and phones - there's no need for the full 'export project' process. This is also invaluable for instantly testing your touch events are working for touchscreen devices.
This should make life much easier when developing for mobiles and tablets! Having Wifi networks makes this especially convenient. Also, there's no limit on how many devices can preview on LAN - if you like, you can have several devices all previewing, and when you preview again they will all simultaneously refresh, allowing you to quickly test on a range of devices.
Don't forget this should work for desktop computers and laptops on the LAN as well. If you have team mates working on a PC next to you, they can use the same process to preview your project on their computer too. This could be helpful in testing on a range of browsers, including those on Mac and Linux.
Note: if you're still having trouble on Windows XP, try this tutorial by user 'cacotigon' for some extra tips.