The trench coat is an iconic staple during transitional seasons that also does a great job of protecting you from the elements. Throw on yours over everything from casual getups to more tailored pieces.
If you're a thrifty spender, check out this Esprit Trench. It's a great product for a reasonable price. You might also be interested in this Esprit Trenchcoat. It's a product you'll love which also won't break the bank.