Xash3D FWGS is a port of the popular Xash3D SDL engine – a rewrite of the Gold source engine – for Android operating systems, which basically lets you play the campaign mode in Half-Life. That said, to do it you'll need to have purchased the game in Steam. The procedure for playing Half-Life using Xash3D FWGS is very simple and comes explained within the app itself. Just copy the 'Valve' file where you've installed Half-Life and paste it into the 'Xash' folder on your Android device (or SD memory card). That done, you can play with no problems. Xash3D FWGS is the easiest way to play Half-Life on Android. Obviously it's a good idea to use some kind of Bluetooth peripheral to play more comfortably but if you don't have one you can always use the touch controls.
Requires Android 2.2 or higher.