WiFi Signal is a Widgets software developed by Yahoo!. After our trial and test, the software is proved to be official, secure and free. Here is the official description for WiFi Signal:

Edit by Marydown: A little digital readout of your WiFi signal strength. It can also show you the name of the network you're connected to, or the signal strength as a raw percentage.

Part of the Yahoo! Widgets 4 installation, but offered here in the event you've lost the Widget.