Grainline Studio's Lark Tee pattern is a classic one with lots of options to make your perfect everyday t-shirt. If that's not enough to get you interested, Jen Beeman recently posted a simple modification to her blog that will turn the t-shirt pattern into a luxurious, open front cardigan.
It really is simple, the modification to the flat pattern taking all of 20 or 30 minutes. Grainline's instructions are always good. I especially appreciated how thorough she was, because knits make me a little nervous. This pattern would be a great way to get comfortable with knits AND make yourself something useful that will get a ton of wear.
You could make it up in a light jersey for a layer piece or heavy sweater knit for something really warm. I made it in a beautiful brushed rayon knit that perfectly drape-y and feels like dancing.