Some ISPs offer hotspots spread around cities where they offer service. If this is the case, then you should be able to access your broadband service using a wireless connection if you are within range of any of these hotspots. If you are instead asking for a way to specifically access your home wireless router from anywhere within the city then, from a practical standpoint, the answer is no, there's not a way to do it.
Even if you were somehow able to boost your signal strength enough that you could pick it up anywhere in the city, the FCC would definitely have something to say about it. All wireless routers share frequencies within a relatively small band. If you were to make your signal that strong, it would likely block any other device trying to use a wifi signal within a significant radius.
As I mentioned above, the most practical solution is to make use of hot spots operated by your ISP. Check with your ISP to see if they offer them in your area and to find out how to access them.