Your site may have great content but if you do not interact with your readers, they will not have any reason to come back. Engaged readers are happy readers, and they are most likely to talk about your site and link back to you, increasing your search engine rankings. Sure, you might hire a full team to update your content many times everyday, but why not offload the content updates to your readers?

Forums
One of the easiest things you can do to increase interaction at your site is to include a forum. There are many free forum software out there: phpBB, punBB, and phorum are some of the most popular and free forum software out there. If you prefer a commercial solution, vBulletin seems to be the defacto. Some web hosts offer an easy forum software installation too (cPanel comes to mind).

Polls and Surveys
Hosting a poll or a survey not only gives you interactivity on your site, it also enables you to gather valuable feedback and statistics from your customers. You can even increase the effectiveness of your poll or survey by using a raffle and offering a real prize. Even if the prize is relatively inexpensive (e.g. an ebook or a discount coupon), other websites will take notice and link to your site.

Calendar
A calendar will give your readers a look into what you are planning in the days, weeks, or months to come. For example, it helps to inform your readers beforehand that you will be organizing a conference or convention within the next coming weeks. Adding a calendar to your site will also increase inter-site linking, and increases  the relevance of the landing pages.

News Ticker
There are many sites that offer news syndication, and you can take advantage of this. Most of the time, a news ticker is placed as a horizontal banner in some non-obtrusive location. The best items to include in a news ticker would just be headlines - it really doesn't work too well with full articles (the vertical space is just too short). You can even include your own news items or links to within your sites.

Random Link
Sometimes, your readers are just too bored or simply looking for something else to do within your site. Offer them a link that will take them to a random article or product. Not only will you increase inter-site linking, you can also verify that all your pages are accessible (watch your web logs for page-not-found requests and see which pages are orphaned). An added benefit is that your customer stays in your site and does not wander off to your competitors.

Blogs and comments
Offer your customers an insight to what your next steps will be. You can also add a comment area on your posts to start off a discussion from a topic. You will find out what your customers are thinking about through their comments.  Additionally, while many search engines don't use links inside comments in calculating your site's rank, other blogs will often link to your blog post.