Samsung’s (infamous) S Voice application is available for all phones again. If you remember, Samsung blocked any phone but the Galaxy SIII from using it, but no blocks can stop a determined modding community. Of course, you must be running a version of Ice Cream Sandwich to use this app, but you do not have to be rooted.
If you are not rooted, you can just install one APK (unless you’re using an early AT&T Android phone without sideloading, then you can use ADB to install it). If you are rooted, there is an alternative method. There are two files to download, and you just move them to their respective locations in the system partition. It’s a little more advanced, but not hard or complicated. Hit the source link for the files, and tell us, do you use S Voice? Or are there better voice assistants on the Play Store?