What do you mean by unique, and what's your budget?
We did ours at Mygatsby.com, and were very happy. Ours did have a few enclosures, but that's because we needed RSVPs for multiple events. You're going to need at least one enclosure...the response card. Ours, though, we did in a pocket format so that all of the enclosures lined up and fit in the single invite, so things would not be lost.
They were close to this design, with our colors and a graphic we chose to reinforce the feel of our wedding, and tied closed with a pretty ribbon.
http://www.mygatsby.com/product.jsp?ProductID=405820
Not "unique" necessarily, but my experience is that truly unique invites tend to cost a lot of money to send out, postage wise.