On my most recent visit to Entertainment Earth, I added a Star Trek item to my cart and I saw this "carrot"*
* "Carrot" is a term for a message in the cart that tells you you're $X away from a promotional offer - usually free shipping. Amazon's Super Saver Shipping is a classic example.
I thought to myself -- whoa, I'm being shown a personalized offer based on my browsing habits! This is HUGE! Then I realized after trying other category items that I had once again confused Star Trek with Star Wars. Hey, it's an honest mistake, right?
But think of the impact this could have if retailers served more relevant incentives in the cart based on a customer's session browsing history or purchase history. By "tagging" customers who click on certain categories or products, search for certain keywords or arrive from search engines through certain keywords - relevant carrot offers could be swapped in lieu of a generic "free shipping" or default giveaway product for all customers.
The tagging concept could also be applied to what kind of products are sitting in the cart. Perhaps you sell name brand and house brand items (Whirlpool and Sears' Kenmore, for example). When your house brands have higher margin, you can offer free shipping at a lower cart threshold than branded items.
These are just a couple ideas for dynamic, personalized merchandising. Really, the possibilities are endless.