"What's My IP?" is a free webapp where you can easily find out what your IP is it is seen by others while you browser the web. You can also find variety of other information about your network and system.
All you have to do is visit the page and it'll instantly tell you your IP address as well as other information which can be inferred from it such as your geographic location and whether you're using a proxy.
A little further down the page you can additionally find information about your user agent, your browser, and your operating system.
IP-Info has a fast, efficient, responsive user interface which works great from all browsers and devices. The site is also intuitive and provides a variety or articles to explain relevant concepts.