Android (rooted): The Google Now Launcher, available on the Nexus 5 and other devices, allows you to say "OK Google" while the screen is on to invoke Google's voice commands. If you're rooted, you can bring this feature to other launchers with an Xposed tweak.
We've talked about the Xposed framework before, so check out our guide to installing it and setting up modules. Then come back here and check out this Xposed module that brings the "OK Google" feature to just about any phone running a third-party launcher.
This all comes thanks to the work XDA Recognised Developer MohammadAG. His module enables "OK Google" listening on third party launchers, on phones running KitKat and above. You can grab it from a direct download or search for "OK Google" in the Xposed Installer.
Very little is needed in the way of setting up, but you'll need to activate the module and restart your handset, as usual. Afterward, you can customise the module to choose which apps have the "OK Google" option. The Xposed module will enable always-on listening, but it will work as long as the screen is active (unlike the always-active listening of the Moto X).