You can set a ringtone to a contact on the Samsung Galaxy Nexus and have a specific ringtone for when a certain person calls you.
1. Open the People app.
2. Choose the person you wish to modify.
3. Press the Menu button.
4. A menu will appear where you can select Set ringtone.
5. Select the ringtone for the contact.
Now when the person calls you from the number specified in their contact information, the ringtone you chose will sound.