Probably the single most enduring, striking image from the entire film is that shot of Andy standing outside Shawshank, shirtless, holding his arms up as the rain washes over him. Not just because he's showing more skin than Rita Hayworth.
The rain could really be symbolic of two things here. First, it could simply represent this new sense of freedom that washes over him as he emerges from the sewer. However, it could also be symbolizing something a little more spiritual.
Andy may not have committed those murders he was put away for, but he's not without a heavy conscience. He admits to Red that he was an awful husband, and drove his wife to have an affair, and feels guilt for her death even if he wasn't the one who pulled the trigger. In a way, then, he's being washed clean of his sins, so he can head out into the world without anything weighing on him, fully prepared to make a fresh start. We know he felt sullied by the shady dealings he had to cooperate with to keep the warden from punishing him. That's all washed away, too.
On the other hand…maybe it just happened to be raining that day of filming. What do we know?