If you want to develop a simple personal page, you can just start by installing a Form mailer script because you may be enthusiastic about your visitors' feedback. Then, later on, add a guest book or text/graphic counter that displays the number of visitors to your site.
If you want to go further you may need a survey to get your visitors opinions and hence should install a Survey or Poll script. And if you want to protect some of your pages for members only, then you can install a password protection script. If you want to sell something online or your business involves eCommerce, then you will be in need of a good shopping cart script, supporting SSL.
I have given a picture of some CGIs which you can find and install in your simple websites according to your wish and need: counter, form, discussion forum, password, web-based free e-mail, shopping cart, chat, download books or scripts, suggest your site to a friend, opinion poll, survey, quiz, database, calendar of events, add a link, free web pages, banner ads, rating, daily quote, site search, search your site and even a search engine.