Android phones have been able to assign customized ringtones to contracts since day one, but if you're in a meeting or a movie, there's no way to know who's calling your silenced phone without whipping it out. Enter ViBe, an app that allows you to assign vibration patterns based on who's calling. ViBe is a free download in the Android Market. We're all adults here - keep it clean in the comments section.
The idea is as simple as the implementation: open up ViBe, select a contact, and choose from 10 baked-in patterns to assign to him or her. If that's not enough, you can create your own pattern with a simple vibration editor. While most people probably won't be able to distinguish a vibration setting for hundreds of contacts, this is a good bit of functionality for the handful of people you talk to the most. You can also assign a pattern to a specific SMS contact, though Google Voice support seems to be lacking.
ViBe is free, but for some quick additional patterns you can purchase additional pre-set patterns for $1.00 each. Though the app crashed initially on my Galaxy Nexus (with custom firmware) it soon worked out the kinks and functioned fine. For additional convenience, you can set the ViBe app to on or off with the built-in widget.