Meetup.com
Some companies just stumble onto a great idea.. I think meetup.com is one of them.
I saw a story about them on CNN a while ago and thought it was a cool idea, and now Joel on Software is organizing a meetup through it. This is a perfect use for them - something that Joel could write some code himself to do on his own site, but meetup.com has maps, membership lists, can list other people in your general area, and so on. It's a good implementation.
My idea of a perfect Internet company is one that involves creating software to provide a service, and letting it take care of itself. eBay is a perfect example.. of course they need network folks and support folks, but really it's a huge service that pretty much runs itself.
And, like eBay, it's a service where we only really need one company. Listing stuff for sale anywhere but eBay means you're going to have a lot less people participating in your auction, so of course almost everyone uses eBay. I think the same will be true of meetup.com - if you want to organize something, that's where most or your potential members will be.
The hard part, of course, is figuring out how to make it so people are willing to pay to use your service. Lots of sites rely on “premium memberships” and advertising (and of course, selling t-shirts). I wonder how well that works in the end..