Cell phones are usually seen as the enemies of good driving, but a new service could make them a motorist's best friend. Nextel has teamed with start-up televigation of Sunnyvale, Calif., to offer on its java-and GPS-enabled phones turn-by-turn onscreen directions along with a computerized voice that speaks the driving instructions out loud. The service includes a business finder to help you locate nearby shops. It can also send your coordinates to another nextel phone via text messaging. Available now, Telenav costs $10 for 10 trips or $20 for 80 trips.