ip, freely
> curl ip3.li
216.73.216.99
ip3.li is designed to be easy to use directly with curl from the command line.
> curl ip3.li
216.73.216.99
A HTTP GET request to ip3.li with a User Agent containing the string curl or WindowsPowerShell will respond only with the plain text IP address of the incoming connection.
curl on PowerShell is slightly different to *nix shells - it returns an object. Therefore, the equivalent command to the above is as follows.
> (curl -s ip3.li).Content
216.73.216.99