In Order of the Phoenix, Harry is given a slip of paper in Dumbledore's handwriting giving him the location of Number 12 Grimauld place. Pettigrew, being secret keeper of the Potter house, was the only one who could give up their address. Yet in the Deathly Hallows, Ron is able to tell Dobby the address to Bill and Fluer's house (Shell Cottage) even though Bill is the secret keeper. Why is that? As I understand, only the secret keeper can tell others the address.
You are right, but up until that moment in the story Shell Cottage hadn't been put under the Fidelius Charm. After Harry and everyone arrive there and the situation about escaping capture from Malfoy Manor was explained to Bill and Fleur (presumably by Ron, Dean, and/or Luna, as Harry was too busy digging Dobby's grave and Hermione, Griphook, and Ollivander were in bad shape) they realized that the Death Eaters now knew that Ron wasn't really ill at home with spattergroit but is actually with Harry, the rest of the Weasleys were not safe so they were immediately notified and went into hiding at Great-Aunt Muriel's home (which was put under the Fidelius Charm as well), and Bill and Fleur took care of matters in their own home.
“. . . lucky that Ginny’s on holiday. If she’d been at Hogwarts they could have taken her before we reached her. Now we know she’s safe too.” He looked around and saw Harry standing there. “I’ve been getting them all out of the Burrow,” he explained. “Moved them to Muriel’s. The Death Eaters know Ron’s with you now, they’re bound to target the family—don’t apologize,” he added at the sight of Harry’s expression. “It was always a matter of time, Dad’s been saying so for months. We’re the biggest blood traitor family there is.” “How are they protected?” asked Harry. “Fidelius Charm. Dad’s Secret- Keeper. And we’ve done it on this cottage too; I’m Secret-Keeper here.
Harry Potter and the Deathly Hallows, Chapter 24 (The Wandmaker)