When you visit a web site, your browser sends various pieces of information to that site. One of them is your IP address (because the server needs it in order to send you the web page you requested). There's a handy site at https://www.whatismyip.com/ which simply echoes this information back to you, which is a really quick and easy way to find out your real IP address.
This can be useful if you're working in an unfamiliar location and you want to know the IP address of the computer you're using, for example. But remember that the result will be the IP address of the router through which your computer is connected to the internet, which probably won't be the same as the one your computer uses internally on the LAN.
The site also tells you a few useful facts about the IP address you're using, such as which company it's registered to. This is handy for checking that your VPN, if you use one, is working correctly.
In a similar vein, here's a web site which tells you your current screen resolution setting. It's a really quick way to check, for example, whether your settings meet the requirements of a program or game that you want to install. Just head to http://www.whatismyscreenresolution.com/ for an instant notification right in your browser, with no need to install or download anything.
So next time you want to install a new game, but the web site says it won't work with displays of less than 1024x768, you'll know precisely how to check that your computer is OK.
Link to What Is My IP Website
Link to Screen Resolution Website