A couple weeks ago I asked for online combined registry sites. We ended up going with myregistry.com (thank you sapphire2009!) and I just want to to tell you all how great it is!
Once you create an account on myregistry.com, you are prompted to add a bookmark to your tool bar that says "add to my registry". Whenever you are on a store website, viewing an item you like, you just click on the bookmark and create a listing for the item in your registry. It is so easy and great for people like my fiance and I who are registering for a variety of random gifts.
MyRegistry.com is not the store that your guests purchase the items from, but it is the registry that the items are listed on. Through the website, your guests are directed to all of the stores that carry the items listed on your registry. Once they are directed to a store, they complete the purchase at the stores website. They can also purchase a listed item in person at a store location and then take it off your registry online later.
I purchase one of the items on our list as a guest, just to check the process. It was easy. The item was removed off the list and arrived at my house.
Ok, enough advertising. I should totally be getting paid for this :)