I'm loyal to my Longchamp totes because they weigh nothing and hold everything, but nylon bags are sticky to carry on a hot day. So I found this light, weather-resistant canvas "City Tote," made by a California company called rockflowerpaper:
It's a good-sized bag; at full-size, it's useful as a beach or market tote. But you can convert it to a smaller shoulder bag by using the hidden snaps on either end, which make it narrower. It has an even trimmer profile if you tuck the sides in, like this:
It has a sturdy base to keep it upright, and soft handles. Inside, there are a couple of useful pockets, plus a snap closure, but no other bells or whistles. It's just a simple, casual bag that lets you to bring along that sweater in case it's chilly, and a hat in case it's sunny... and some sunblock, and a magazine.... It's good for farmer's market trips, too.
Shipping is free on their website for orders over $25. This bag comes in many patterns and colors, and they have a few other styles as well.