Google Now is one of the coolest features of Android 4.1 Jelly Bean, letting you do nearly anything with your voice and get real-time updates on the things important to you. If you don't have Jelly Bean, you can still get Google Now for Ice Cream Sandwich with a simple root tweak.
The folks over at XDA Developers have put together a flashable ZIP file that contains Google Now in its entirety, with voice search and real-time updates in tow. All you need is a phone that is rooted and running an Ice Cream Sandwich-based ROM. Download the ZIP to your phone and flash it from recovery mode just like you would an Android ROM, and you'll be good to go. Check out the thread at XDA for step-by-step instructions.
Google Now Flashable Zips with Noob-Friendly Instructions [XDA Developers via Droid Life]