Android (rooted): Mirroring your Android screen to Chromecast is great for showing off photos and videos, but it's only supported by a handful of devices. Mirror Enabler is an experimental tool from r3pwn that unlocks the feature for many older handsets.
You need to have a rooted handset running KitKat with the Chromecast app installed. Download a copy of Mirror Enabler and fire it up -- when you look for it in your list of apps, note that it is labelled #MirrorEnabler and will appear towards the top of the list.
Tap the toggle to switch it to Enabled and launch the Chromecast app. Tap the menu and select the Cast Screen option and whatever you see on the screen of your handset will be broadcast to your Chromecast. An icon is visible in the notification bar whenever casting is enabled, and you can disabled it at any time by tapping the Disconnect option.
It's not guaranteed to work with all KitKat handsets, but it's certainly worth a try. The forum thread linked below has a list of devices the tool is confirmed to work with, so you might want to check that out first to see if your phone has been tested.