Windows only: There may be many reasons why people have always felt a need to hide their actual IP addresses. The intentions may not be always malicious. It may be to protect privacy, for overcoming censorships, to browse anonymously, to bypass Rapidshare download limit, circumvent the Megaupload country slot limit etc.
So people use proxy servers. When one use a proxy server, the outside world would see the address of the proxy server and not the user's actual IP address.
NotMyIp is an application that makes it dead simple to mask one's IP address. This is a freeware version of the popular premium product - Anonymity Gateway. NotMyIp doesn't require any manual configurations. It automatically configures all installed browsers like Firefox, IE, Opera and Google Chrome to use high quality US proxies. This takes the pain away from the user as he longer need to search for free proxies in the internet and configure the browsers manually.
Not My Ip is a free for non-commercial use tool, available for Windows only.