Somen are thin Japanese noodles made from wheat flour; they're available in the Asian section of many grocery stores. Vermicelli or thin spaghetti can be used instead.
Make dressing: In a medium bowl, whisk together ginger, lime zest, lime juice, vinegar, salt, and pepper. Whisking constantly, add oil in a steady stream until mixture is emulsified; set aside.
Bring a large saucepan of water to a boil. Add noodles, and cook until al dente according to package instructions, about 6 minutes. Drain in a colander, and transfer noodles to a serving bowl.
Add dressing, scallions, and peanuts to noodles; toss to combine. Serve warm, at room temperature, or chilled, and garnish with additional scallions and peanuts.