document updated 13 years ago, on Oct 25, 2011

There are various ways to have different ringtones for:

different types of events (eg. incoming SMS versus incoming phone call)
different people
different phone numbers (i.e. if you have a Google Voice number, people can either call that, or call your cellphone directly)
many other combinations (different time of day, etc)

This is available by default. Go to the person's contact page, press Menu > Options > Ringtone.

Apps that can help with this: