1. Humor. Add a cute or funny quote about love or marriage. A few of my favorites are: "Marriage is when you get to keep your girlfriend and don't have to return her to her parents" (6-yr old boy), "Love is what makes you smile even when you're tired" and " I was nauseous and tingly all over. I was either in love or I had smallpox" (Woody Allen).
2. Photos. I love photos on everything and they add so much personality to your invite. And it'll look great in an album with all the wedding photos later. You can of course scan a photo (or use one taken with a digital camera) and use that, but also consider things like photo transfers, using a strip of photos of the two of you from a photo booth, one taken with a fisheye lens, or fun Polaroid's. The possibilities are endless.
3. Recordable cards. Hallmark has had great success with their line of sound greeting cards, and now, several manufacturers are making cards where you can record your own message. At Snapfish, you can upload your own photo or design (save it as a jpg), add text and then add your voice at home.
4. Magazine Cover. At MagMyPic, you can upload a photo and add it to any of their fake magazine covers (like "TruLove" or "Bride") and voila! Your own cover. At the moment, you have to stick with the text that's already there, but customizable headlines and text is in the works.
5. Posters. Poster Bride in Seattle make hand-printed serigraphs of your invitation, from a standard A7 to 19x25 inch silk-screened posters. Nobody will be able to misplace that invitation!
6. Pingg. I know many people say it's declasse to invite people online, and for a wedding you probably want to go the traditional snail mail route. Or why not do a combination of both? At Pingg, a snazzier version of Evite, you can build your guest list, create your invitation, allow your guests to RSVP online, and even add a gift registry. Why not send the "Save-the-Date" by email, the invitation by regular mail, and ask people to RSVP online? You'll use a lot less paper, save money on stamps, and your guests won't have to go looking for a mailbox to respond. Hey, who's saving the environment now?