The most innovative ways, might also actually be the least innovative ways:

* Send them hand written thank you notes for their business.

* Interview them about their successes, and highlight them in your blog/on your site.

* Where appropriate, send them business from other customers

* With their permission, talk about them in your PR and interviews even more than you talk about your product

* Tell them whenever someone signed up because of their personal referral

* Actually answer the phone

The best way to delight clients is to under-promise and over-deliver. Do something unexpected and valuable (to them), and they'll love you forever. :)

Care. Actually, really care. Treat them like they are your only or most important client.

Airlines and other crappy companies are lowering this bar but Zappos and other amazing companies are raising it.

Also - a good post from Seth on why "answering on the first ring" might be as important as delighting your customers in a more innovative way.

http://sethgodin.typepad.com/seths_blog/2006/03/waiting_in_line.html

Adii's answer is a great one.

I travel alot so I like to invite customers to a lunch or a coffee if I'm in their area to find out if we can help them in any other way or to hear feedback on what they don't like or what we can do to be better. Having them involved on a meaningful level where you can share your vision with them creates the most meaningful and valuable relationship you could hope to have with a customer. It goes a long way and creates evangelists for your brand. That's ultimately why you want to delight your customers.

I recently just sent this email to Steve at Kontagent, one of our customers, as he was a customer reference for an investor in our company:

Steve,

Thanks again for the time with Eric. We're gonna know Wednesday on their decision. They said it went well and if we get approval, I'll be in town the 6th or so. I'd like to buy you lunch or a coffee if you're interested.

And as good as your feedback has been, could you tell us what you don't like about Ecquire?

Thanks again, Steve and let me know on both items.

Best way is simple: demonstrate you understand their problem, "get" them and are committed to making that part of their life better. Show them that with everything you do, say and build and they will love you and tell their friends about you.

Excellent answer from Thomas.

The book "Reinventing the Wheel" by Chris Zane is an inspiring read for someone keen to out-innovate their competition by offering more delight to customers. Contains a few blue ocean strategies that may inspire your thinking.