AlertIP is a Firefox addon, can show Site IP of the currently loaded web page in Firefox. When you switch frequently IP addresses using your /etc/hosts you need to check, what IP does the domain translates to - is it your development site or the real production site?
There is a outstanding extension ShowIP which does it, but it shows the IP all the time and sometimes suffer from DNS caching issues. There is some computing overhead as well. Unlike AlertIP which is dead simple - it show the IP of the currently loaded web page only when asked for.

Requirements:
* Firefox

The license of this software is Free, you can free download and free use this browser plug-ins software.