Hotspot Shield is a privacy and anonymity tool for Mac and Windows. It is a VPN service that allows you to effectively hide your IP address and web browsing activity. When you "get behind the shield", your IP address will be hidden, and you will get a new one that other users on the Internet will see. This new IP address is based in the United States, so you can use this tool to connect to websites and services that restrict access to users outside the United States.
Hotspot Shield has been around for quite some time now, and it works equally as well on Windows and Mac OS X. When you install it, an icon will appear on the top menu bar. When the service is disconnected, the icon will be red, and it will go to green upon a successful connection. In my time with it, I haven't been unable to connect to the service. Most websites that restrict access work when connected to HotSpot Shield. The only problem with it is speed. You are not going to be able to max out your speed while connected to the service. Web browsing and similar activities don't pose a problem, but if you want to stream high-definition content using this tool, you will find that the service can be a bit slow.
All in all, Hotspot Shield is a free tool that works very well. It is not the fastest, but it should be enough for normal usage.