What is CGI?

CGI stands for Common Gateway Interface and is a way in which your browser is allowed to communicate with programs running on a server. Baffled?

Simply it means: 

The front end of a portal between which your browser and the server communicate, that is accessible to anyone, regardless of their software.

Where to get it:

Don't ask me how to write it for yourself because I don't know, BUT I can link you to some good sites that offer free CGI.

CGI City

Some good scripts that you can download, including one that takes visitors to a good-bye page before actual redirection.

Hosted Scripts

This one has loads of Brilliant Remotely hosted CGI scripts from guest books to FFA pages. (If you demo the features you can add your site to some promotional scripts like FFA page and Top site list for some free and easy website promotion)

CGI Resource Index

This is a huge collection of CGI whatever you want, you'll find it here.