Here is a random sample of some companies who I think are doing a great job engaging with their customers.
Cluttercontrolfreak.com
This site is Stacks and Stacks' blog devoted to the topic of organization. Admittedly, I'm not really familiar with the parent company (and I'm only a wannabe organization freak). However, what I love about this blog is that it's not a glorified Stacks and Stacks ad, but is a genuine resource for people who are crazy for organization. When I first came across it, almost accidentally, I was impressed that this seemingly modest, not overly impressive (that sounds harsh, but that's what I thought) company decided to connect with this niche market in an engaging way. I like that.
J Crew
I LOVE J Crew. Even if they sometimes make it hard, like when they tried to push plaid pants a couple of years ago. Anyway, they are so together in their marketing efforts. For me, it all starts with a direct e-mail notification (how did they know I'd cave last month and buy a sweater because they gave me free shipping?!). Then it moves on to the personal shopper experience (a new service they are promoting, and I ate it up, even if I did spend [a ton] more than I was going to). Down to the J Crew debossed notecards at their catalogue phone desk, where I can jot down my order number (no talls in the retail stores, unfortunately). I can name a dozen reasons why J Crew excels in the user experience, but I encourage you to check out their website and find out why yourself.
TheBabyWearer.com
I will be the first to admit this site is ugly and not intuitive. But despite the ugliness, thebabywearer.com has an absolutely thriving, cult-like user forum. Women interested in baby wearing spend hours, racking up hundreds, even thousands, of posts, discussing baby carriers and the joys of babywearing. Beyond product reviews and debates over the best carriers, there is an extremely active for sale or trade forum where mama's all over world sell to each other via pay pal accounts. What I love so much about this site is how involved and enthusiastic the members are, and how so much of the content is user generated. There was a very brief period where I was part of the cult, and this site dispelled any ideas I once had that social networking was either for the tech crowd or for kids. Anyone and everyone can find a place online to belong.