I agree with a few posters about Kate Spade - I know that she has been trendy for a while, and people don't really consider her line "designer", but I find that some of the pieces are very unique with a great retro feel to them. I love purchasing the more interesting bags every season and love the summer straw/wicker lines.
Another brand, more expensive, that I think is underrated is Tod's. I know that a few people on the forum LOVE them, but not much discussion in general. Great quality bags, and classic styling.