I love twinsets -- matching cardigan and shell are perfect for business casual days and if the need for a full-suit arises unexpectedly, the cardigan can be switched for a suit jacket. Unfortunately most nice twinsets incorporate silk and/or wool, especially in the winter. But JCrew's new twinset is made from cotton, nylon, and spandex. And it comes in 13 colors. Excellent!
The cardigan (pictured in navy) has three-quarter sleeves (great for summer!) and is $64 (here). The shell has a rib trim at the armholes, which nicely hides pesky underwear, and is $44 (here; note that shell is available online and via catalog only).