What are some of the better online business ideas or concepts you've seen?

Answer:
This is a good topic and I may rewrite this answer

regularly. I read tons of startup and tech blogs everyday and the majority of the concepts I read about are good but they are not great. Over the past 9 years I have seen lots of trends come and go. From instant message to map mashups there have been quite a few lemons.

My basis for this answer isn't strictly which concepts are the most unique but which concepts have been the most successful in their niche. One company that comes to my mind as being a great concept is Zappos. Zappos.com is an online company that sells shoes. Yes, you stick your feet into your monitor and try shoes on virtually in their store. Not really, don't try that at home as you'll likely be needing a new monitor. To the blind eye this concept seems like a failure waiting to happen. Shoes are something you have to try on in order to find the right fit. Zappos understands this so they've made it super easy to order more than one pair and send them back. It's really a fantastic system and logistically a huge feat. I've purchased shoes from Zappos and I have to say it was very enjoyable. So you order several pairs of shoes, maybe different sizes or different brands. You get your shoes in less than 2 days in most instances. You try the shoes on and keep the ones you like or that fit. You put the others back in the boxes, take them to the post office and send them back. Zappos covers all shipping costs and I've found the selection and price to be very good. Big thumbs up for finding a niche that normally would be considered a difficult product to sell online and making it easy. I recently read that Zappos is forecasting $1billion in sales for 2008 so it looks like I'm not the only one that likes to buy shoes online.

Another concept I really like is del.icio.us . Del.icio.us or delicious for now on as is a bookmarking site. You add a little button to your browser and when you find a site or page you want to save you simply click on the "post to del.icio.us" button. You type in a description of the page and tag it with keywords. You now have as many bookmarks as you want, all categorized and accessible from anywhere. You can be in Russia on someone else's computer and access your bookmarks. Delicious makes it easy to import your existing bookmarks from your browser. Acquired by Yahoo! I am unsure of revenues but this is one slick, simple concept that proves to provide useful features.

There are dozens of other concepts that deserve merit so stay tuned for updates to this answer.