|When you said you could "carry a tune," I thought you meant...|
There are many variations on bicycle baskets, from the plastic milk crate zip-tied to a rack all the way up to high-end contrivances with built-in lock holsters, but one of the most common and most useful is the inexpensive wire basket. A handlebar-mounted basket makes it easy to toss in a few things, whether a book or a bag of groceries, and go without thinking too much about it.
On most bikes, handlebar-mounted baskets like this one are a great for loads up to about 10 pounds. Beyond that, on many bikes, they will start to affect the handling (although how much they affect you depends on a lot of factors, many riders are comfortable carrying quite a bit more weight, and some bikes are specifically designed to haul a heavy front load). If you carry multiple bags of groceries at a time, or larger objects, adding rear mounted baskets might make hauling more weight easier, as weight over the rear wheel affects steering less dramatically than weight on the handlebar.
|With larger rear baskets, be sure you have enough room so that your heel doesn't bang into the basket when you pedal.|
I'm particularly a fan of inexpensive wire baskets like those made by Wald. Not only are they inexpensive, but they're often sturdier and more reliable than flashier options. I've especially found wicker baskets to be more fashion than function, as they don't mount as securely and don't tend to have much capacity in proportion to what they weigh.
Baskets do have their disadvantages. They don't offer any protection from weather, and, of course, smaller objects can easily fall out unless they're in some sort of bag. Overall though, for carrying small loads by bicycle, a basket is a cheap and effective option that has served the commuting cyclist for decades.