If you live in a low-crime area and don't have a very expensive bike, I'd recommend an OnGuard U lock. They sell for under $30 and are very strong. Get the smallest size that will fit over your bike and a parking meter--this will make it difficult for a thief to fit a pry bar in there to open the lock with force. These locks don't test well against power tools, but it doesn't sound like most thieves in your area will have them.
I'd advise against a cable lock. These can be clipped with bolt cutters, which are commonly carried by bike thieves everywhere.
There are a lot of resources online with lock reviews, where magazines like Consumer Reports have timed how long it takes thieves to break all different kinds of locks with all different kinds of tools. Here's one:
http://www.consumersearch.com/bicycl...est-bike-locks Since all locks are breakable/pickable, your best bet is to make thief take more time than he is willing to commit (balanced with your budget and the amount of extra weight you're willing to carry around).