My favorite is the Merona Ottoman Trench with trim in deep navy -- although, it might not go with every outfit you wear to work. Medium weight; perfect for cooler spring days; made from cotton with polyester lining. I especially enjoy the two rows of contrast piping down the back and the funnel neck. The trench is $54.99; here.
For a more classic look, I suggest the Merona Long Double-Breasted Trench. It comes in the pictured black and off white. Made from cotton and nylon with polyester lining. The trench is on sale for $27.99 (originally $34.99 -- 20% off). Find the black here and the off white here.