One of the key new features of Jelly Bean is Google Now, the voice assistant that learns from you and provides information it thinks will be useful. Many ICS users have been wanting this feature for themselves, but while there have been mods available for some time they have lacked full voice functionality.
Now XDA Senior member SilentStormer has combined the ICS voice-to-text engine with the Google Now app to provide a mod that works with most rooted versions of ICS.
Check out this Google Now XDA Thread for full details on how to install this mod. I’ve tried it myself this morning and can confirm it works great (but results may vary – do a backup before you start).