If you have ever setup a home network you probably run into Windows File and Printer Sharing, the feature of Windows that enables users to share files and printers over a local network.
This feature, although useful, is also very dangerous if improperly configured. For example older versions of XP did share the root of your hard disk (this is one of the so called Administrative Shares). Normally you would need the administrator password to gain access to these shares from a remote host but, if I remember correctly, those old versions of XP did share them without a password if you hadn't set one for the Administrator account. Newer versions (SP2 or greater perhaps) don't leave the Administrative Shares unprotected but still many machines are misconfigured in other ways.
In fact some machines are so badly misconfigured that you can see their shared folders from the Internet. For example Dr.X has a computer that connects to the Internet with an old ADSL modem and shares this connection to the rest of his house. He also shares a folder with personal files in his internal network which accidentally is also shared over all the network interfaces of his computer, one of which is the ADSL modem which connects to the Internet.
Before some years you could do a port 445 (the port used by Windows for Windows Shares) scan (portsweeping as Wikipedia tells me) in your neighboring IP addresses and find lots of misconfigured hosts. Now these incidents have been limited by the adoption of the otherwise crappy Windows Vista and the usage of otherwise annoying firewalls. But still "victims" can be found.
Steal Something Today (SST) is a simple portscanner specialized in finding misconfigured hosts. You simply run it, press two buttons and wait until computers start popping up in explorer windows. SST will start a bidirectional scan starting from your IP address (or any other address) and it will list all hosts with port 445 open. By default it will automatically open these hosts with windows explorer. These host will luckily need a password but I always find many hosts sharing directories completely unprotected.
SST is completely portable. It stores its settings inside an INI file in its directory. It is a VB6 application using the Winsock control which is included in the zipfile and in most cases it will simply run by double clicking the exe. I haven't tested it on Vista and I don't care doing so. :-P Also it probably doesn't work on Linux with Wine (but you can still run it in a virtual machine).
As for the legality of the program: First of all by using it you agree that you are the solely responsible for what you do with it. The program itself is not illegal, not even by the means of DMCA or other equally stupid laws since it does not break any kind of security system. However portscanning might be prohibited by the terms of usage that your ISP made you agree on. Anyway, that paragraph was not necessary, I just wanted to show how much I hate the DMCA. (also copyrights in general) :-)
Download SST by clicking here. (VB6 sources included, read the license terms)