1.) Forums: Having a forum is a great way to provide interaction on your website. Users can share ideas, trade links and discuss whatever topic is relevant to your site. It is also a great way for users with similar interests (such as whatever your site is about) to get to know each other. Building a strong community around your website will not only be a rewarding experience, it will also aid in the success of your website. There are several languages you can create a forum with, but PHP is probably the most popular. I personally love phpBB, but there are lots of other very nice free PHP based forums available to download on the internet, and one is sure to be a good fit for your site.
2.) Blogs: Don't just write it, blog it. Blogs are a great way to make your website interactive. For one, it gives readers a way to subscribe to your site (your RSS feed) without the intrusiveness of being on an email list. It also gives the users a way to give feedback on what you've written though the comments section on each blog post. Like a forum, blogs are also often written in PHP. One of my favorites is WordPress, but there are many others to choose from.
3.) Polls: Ask for an opinion. Who doesn't like to vote in a poll? Having a poll on your website can provide useful information about your site's readers, as well as being a fun and interactive way for the users to express themselves on your website. By rotating the poll daily or weekly, you can get input on a variety of topics. You can mix fun polls like "Who's your favorite ___" with useful stuff like "What would you like to see on this site?" Have a look at this simple poll script written in PHP with MySQL.
4.) Ratings: Was this good? Having a ratings system on your site helps show your readers that you care about how your product or articles are perceived. By allowing users to rate articles you know what needs to be fine tuned, and what is already a big hit with your readers. You could have something as simple as "Was this article helpful" on the bottom of each page, to something as complex as user generated ratings and reviews of products you may sell. Regardless of how far you want to take the rating system, it is a good way to make your website interactive, while also providing you with valuable feedback.
5.) Social Sites: Yes, I mean Facebook and Twitter. The fact of the matter is, a lot of people use these social websites. It can only be helpful for you to be where the people are, and for many people "going online" means "checking my Facebook." Fortunately it's easy to integrate both Facebook and Twitter directly into your website, so fans can follow you right from your home page.